diff --git a/facets.info.yml b/facets.info.yml
index 24fce91..15d688f 100644
--- a/facets.info.yml
+++ b/facets.info.yml
@@ -7,3 +7,4 @@ configure: facets.overview
 test_dependencies:
   - search_api:search_api (1.x-dev)
   - drupal:views
+  - drupal:system (>=8.2)
diff --git a/src/Form/FacetForm.php b/src/Form/FacetForm.php
index ac2a43c..22812bd 100644
--- a/src/Form/FacetForm.php
+++ b/src/Form/FacetForm.php
@@ -4,7 +4,6 @@ namespace Drupal\facets\Form;
 
 use Drupal\Component\Utility\Html;
 use Drupal\Core\Entity\EntityForm;
-use Drupal\Core\Entity\EntityTypeManager;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\facets\Processor\ProcessorInterface;
@@ -13,6 +12,7 @@ use Drupal\facets\UrlProcessor\UrlProcessorInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Drupal\facets\Widget\WidgetPluginManager;
 use Drupal\facets\Processor\SortProcessorInterface;
+use Drupal\Core\Form\SubformState;
 
 /**
  * Provides a form for configuring the processors of a facet.
@@ -229,13 +229,11 @@ class FacetForm extends EntityForm {
           '#access' => !$processor->isHidden(),
         );
 
-        $processor_form_state = new SubFormState(
-          $form_state,
-          ['facet_settings', $processor_id, 'settings']
-        );
-        $processor_form = $processor->buildConfigurationForm($form, $processor_form_state, $facet);
+        $subform = &$form['facet_settings'][$processor_id]['settings'];
+        $processor_form_state = SubformState::createForSubform($subform, $form, $form_state);
+        $processor_form = $processor->buildConfigurationForm($subform, $processor_form_state, $facet);
         if ($processor_form) {
-          $form['facet_settings'][$processor_id]['settings'] = array(
+          $subform = array(
             '#type' => 'details',
             '#title' => $this->t('%processor settings', ['%processor' => (string) $processor->getPluginDefinition()['label']]),
             '#open' => TRUE,
@@ -252,7 +250,7 @@ class FacetForm extends EntityForm {
               ),
             ),
           );
-          $form['facet_settings'][$processor_id]['settings'] += $processor_form;
+          $subform += $processor_form;
         }
       }
     }
@@ -285,13 +283,11 @@ class FacetForm extends EntityForm {
           '#access' => !$processor->isHidden(),
         );
 
-        $processor_form_state = new SubFormState(
-          $form_state,
-          array('facet_sorting', $processor_id, 'settings')
-        );
+        $subform = &$form['facet_sorting'][$processor_id]['settings'];
+        $processor_form_state = SubformState::createForSubform($subform, $form, $form_state);
         $processor_form = $processor->buildConfigurationForm($form, $processor_form_state, $facet);
         if ($processor_form) {
-          $form['facet_sorting'][$processor_id]['settings'] = array(
+          $subform = array(
             '#type' => 'container',
             '#open' => TRUE,
             '#attributes' => array(
@@ -307,7 +303,7 @@ class FacetForm extends EntityForm {
               ),
             ),
           );
-          $form['facet_sorting'][$processor_id]['settings'] += $processor_form;
+          $subform += $processor_form;
         }
       }
     }
@@ -479,21 +475,15 @@ class FacetForm extends EntityForm {
     // Iterate over all processors that have a form and are enabled.
     foreach ($form['facet_settings'] as $processor_id => $processor_form) {
       if (!empty($values['processors'][$processor_id])) {
-        $processor_form_state = new SubFormState(
-          $form_state,
-          array('facet_settings', $processor_id, 'settings')
-        );
-        $processors[$processor_id]->validateConfigurationForm($form['facet_settings'][$processor_id], $processor_form_state, $facet);
+        $processor_form_state = SubformState::createForSubform($form['facet_settings'][$processor_id]['settings'], $form, $form_state);
+        $processors[$processor_id]->validateConfigurationForm($form['facet_settings'][$processor_id]['settings'], $processor_form_state, $facet);
       }
     }
     // Iterate over all sorting processors that have a form and are enabled.
     foreach ($form['facet_sorting'] as $processor_id => $processor_form) {
       if (!empty($values['processors'][$processor_id])) {
-        $processor_form_state = new SubFormState(
-          $form_state,
-          array('facet_sorting', $processor_id, 'settings')
-        );
-        $processors[$processor_id]->validateConfigurationForm($form['facet_sorting'][$processor_id], $processor_form_state, $facet);
+        $processor_form_state = SubformState::createForSubform($form['facet_sorting'][$processor_id]['settings'], $form, $form_state);
+        $processors[$processor_id]->validateConfigurationForm($form['facet_sorting'][$processor_id]['settings'], $processor_form_state, $facet);
       }
     }
 
@@ -538,10 +528,7 @@ class FacetForm extends EntityForm {
         $new_settings['weights'] = $values['processors'][$processor_id]['weights'];
       }
       if (isset($form[$form_container_key][$processor_id]['settings'])) {
-        $processor_form_state = new SubFormState(
-          $form_state,
-          array($form_container_key, $processor_id, 'settings')
-        );
+        $processor_form_state = SubformState::createForSubform($form[$form_container_key][$processor_id]['settings'], $form, $form_state);
         $processor->submitConfigurationForm($form[$form_container_key][$processor_id]['settings'], $processor_form_state, $facet);
         $new_settings['settings'] = $processor->getConfiguration();
       }
diff --git a/src/Form/SubFormState.php b/src/Form/SubFormState.php
deleted file mode 100644
index 39f75a7..0000000
--- a/src/Form/SubFormState.php
+++ /dev/null
@@ -1,842 +0,0 @@
-<?php
-
-namespace Drupal\facets\Form;
-
-use Drupal\Component\Utility\NestedArray;
-use Drupal\Core\Form\FormInterface;
-use Drupal\Core\Form\FormState;
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Url;
-use Symfony\Component\HttpFoundation\Response;
-
-/**
- * Represents the form state of a sub-form.
- */
-class SubFormState implements FormStateInterface {
-
-  /**
-   * The keys which should be inherited as-is from the main form state.
-   *
-   * @var bool[]
-   */
-  protected static $inheritedKeys = array(
-    'build_info' => TRUE,
-    'rebuild_info' => TRUE,
-    'rebuild' => TRUE,
-    'response' => TRUE,
-    'redirect' => TRUE,
-    'redirect_route' => TRUE,
-    'no_redirect' => TRUE,
-    'method' => TRUE,
-    'cache' => TRUE,
-    'no_cache' => TRUE,
-    'triggering_element' => TRUE,
-  );
-
-  /**
-   * The form state of the main form.
-   *
-   * @var \Drupal\Core\Form\FormStateInterface
-   */
-  protected $mainFormState;
-
-  /**
-   * The keys that lead to the desired sub-form in the main form.
-   *
-   * @var string[]
-   */
-  protected $subKeys;
-
-  /**
-   * Internal storage for the sub-state, writing into the main form state.
-   *
-   * @var array
-   */
-  protected $internalStorage;
-
-  /**
-   * The values of the sub-form.
-   *
-   * @var array
-   */
-  protected $values;
-
-  /**
-   * The input of the sub-form.
-   *
-   * @var array
-   */
-  protected $input;
-
-  /**
-   * Constructs a SubFormState object.
-   *
-   * @param \Drupal\Core\Form\FormStateInterface $main_form_state
-   *   The state of the main form.
-   * @param string[] $sub_keys
-   *   The keys that lead to the desired sub-form in the main form.
-   */
-  public function __construct(FormStateInterface $main_form_state, array $sub_keys) {
-    $this->mainFormState = $main_form_state;
-    $this->subKeys = $sub_keys;
-    $sub_state = &$main_form_state->get('sub_states');
-    if (!isset($sub_state)) {
-      $sub_state = array();
-    }
-    $this->internalStorage = &$this->applySubKeys($sub_state);
-    if (!isset($this->internalStorage)) {
-      $this->internalStorage = array();
-    }
-    $this->values = &$this->applySubKeys($main_form_state->getValues());
-    if (!is_array($this->values)) {
-      $this->values = array();
-    }
-    $this->input = &$this->applySubKeys($main_form_state->getUserInput());
-    if (!is_array($this->input)) {
-      $this->input = array();
-    }
-  }
-
-  /**
-   * Applies the sub-form's array keys to the given original array.
-   *
-   * @param array $original
-   *   The original array, belonging to the main form.
-   *
-   * @return array
-   *   The corresponding array for the sub form, as a reference.
-   */
-  protected function &applySubKeys(array &$original) {
-    return NestedArray::getValue($original, $this->subKeys);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getCompleteForm() {
-    return $this->applySubKeys($this->mainFormState->getCompleteForm());
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setCompleteForm(array &$complete_form) {
-    $sub_form = &$this->applySubKeys($this->mainFormState->getCompleteForm());
-    $sub_form = $complete_form;
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function loadInclude($module, $type, $name = NULL) {
-    return $this->mainFormState->loadInclude($module, $type, $name);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getCacheableArray() {
-    return array();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setFormState(array $form_state_additions) {
-    foreach ($form_state_additions as $key => $value) {
-      $this->set($key, $value);
-    }
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setResponse(Response $response) {
-    $this->mainFormState->setResponse($response);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getResponse() {
-    return $this->mainFormState->getResponse();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setRedirect($route_name, array $route_parameters = array(), array $options = array()) {
-    $this->mainFormState->setRedirect($route_name, $route_parameters, $options);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setRedirectUrl(Url $url) {
-    $this->mainFormState->setRedirectUrl($url);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getRedirect() {
-    return $this->mainFormState->getRedirect();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setStorage(array $storage) {
-    $this->internalStorage = $storage;
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getStorage() {
-    return $this->internalStorage;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &get($property) {
-    if (isset(self::$inheritedKeys[$property])) {
-      return $this->mainFormState->get($property);
-    }
-    $value = &NestedArray::getValue($this->internalStorage, (array) $property);
-    return $value;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function set($property, $value) {
-    if (isset(self::$inheritedKeys[$property])) {
-      $this->mainFormState->set($property, $value);
-    }
-    else {
-      $this->internalStorage[$property] = $value;
-    }
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function has($property) {
-    return isset(self::$inheritedKeys[$property])
-        || array_key_exists($property, $this->internalStorage);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setBuildInfo(array $build_info) {
-    $this->mainFormState->setBuildInfo($build_info);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getBuildInfo() {
-    return $this->mainFormState->getBuildInfo();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function addBuildInfo($property, $value) {
-    $this->mainFormState->addBuildInfo($property, $value);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getUserInput() {
-    $user_input = &$this->mainFormState->getUserInput();
-    return $this->applySubKeys($user_input);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setUserInput(array $user_input) {
-    $old = &$this->getUserInput();
-    $old = $user_input;
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getValues() {
-    return $this->values;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getValue($key, $default = NULL) {
-    if ($this->hasValue($key)) {
-      return $this->values[$key];
-    }
-    return FALSE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValues(array $values) {
-    $this->values = $values;
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValue($key, $value) {
-    $this->values[$key] = $value;
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function unsetValue($key) {
-    unset($this->values[$key]);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function hasValue($key) {
-    if (isset($this->values[$key])) {
-      return TRUE;
-    }
-    return FALSE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isValueEmpty($key) {
-    if (empty($this->values[$key])) {
-      return TRUE;
-    }
-    return FALSE;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValueForElement(array $element, $value) {
-    $this->mainFormState->setValueForElement($element, $value);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public static function hasAnyErrors() {
-    return FormState::hasAnyErrors();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setErrorByName($name, $message = '') {
-    $this->mainFormState->setErrorByName(implode('][', $this->subKeys) . '][' . $name, $message);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setError(array &$element, $message = '') {
-    $this->mainFormState->setError($element, $message);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function clearErrors() {
-    $this->mainFormState->clearErrors();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getErrors() {
-    return $this->mainFormState->getErrors();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getError(array $element) {
-    return $this->mainFormState->getError($element);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setRebuild($rebuild = TRUE) {
-    $this->mainFormState->setRebuild($rebuild);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isRebuilding() {
-    return $this->mainFormState->isRebuilding();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setInvalidToken($invalid_token) {
-    $this->mainFormState->setInvalidToken($invalid_token);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function hasInvalidToken() {
-    return $this->mainFormState->hasInvalidToken();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function prepareCallback($callback) {
-    return $this->mainFormState->prepareCallback($callback);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getFormObject() {
-    return $this->mainFormState->getFormObject();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setFormObject(FormInterface $form_object) {
-    $this->mainFormState->setFormObject($form_object);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setAlwaysProcess($always_process = TRUE) {
-    $this->mainFormState->setAlwaysProcess($always_process);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getAlwaysProcess() {
-    return $this->mainFormState->getAlwaysProcess();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setButtons(array $buttons) {
-    $this->mainFormState->setButtons($buttons);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getButtons() {
-    return $this->mainFormState->getButtons();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setCached($cache = TRUE) {
-    $this->mainFormState->setCached($cache);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isCached() {
-    return $this->mainFormState->isCached();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function disableCache() {
-    $this->mainFormState->disableCache();
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setExecuted() {
-    $this->mainFormState->setExecuted();
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isExecuted() {
-    return $this->mainFormState->isExecuted();
-  }
-
-  /**
-   * {@inheritdoc}
-   *
-   * @todo What are groups? Is this the way to handle them in a sub-form?
-   */
-  public function setGroups(array $groups) {
-    $this->mainFormState->setGroups($groups);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getGroups() {
-    return $this->mainFormState->getGroups();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setHasFileElement($has_file_element = TRUE) {
-    $this->mainFormState->setHasFileElement($has_file_element);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function hasFileElement() {
-    return $this->mainFormState->hasFileElement();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setLimitValidationErrors($limit_validation_errors) {
-    $add_subkeys = function (array $path) {
-      return array_merge($this->subKeys, $path);
-    };
-    $limit_validation_errors = array_map($add_subkeys, $limit_validation_errors);
-    $this->mainFormState->setLimitValidationErrors($limit_validation_errors);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getLimitValidationErrors() {
-    $limit_validation_errors = $this->mainFormState->getLimitValidationErrors();
-    if (is_null($limit_validation_errors)) {
-      return NULL;
-    }
-    $return = array();
-    $sub_keys_count = count($this->subKeys);
-    foreach ($limit_validation_errors as $path) {
-      if (array_slice($path, 0, $sub_keys_count) == $sub_keys_count) {
-        // If the whole sub-form is included, it is the same (for the sub-form)
-        // as if there was no limitation at all.
-        if (count($path) == $sub_keys_count) {
-          return NULL;
-        }
-        $return[] = $path;
-      }
-    }
-    return $return;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setMethod($method) {
-    $this->mainFormState->setMethod($method);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setRequestMethod($method) {
-    $this->mainFormState->setRequestMethod($method);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isMethodType($method_type) {
-    return $this->mainFormState->isMethodType($method_type);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValidationEnforced($must_validate = TRUE) {
-    $this->mainFormState->setValidationEnforced($must_validate);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isValidationEnforced() {
-    return $this->mainFormState->isValidationEnforced();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function disableRedirect($no_redirect = TRUE) {
-    $this->mainFormState->disableRedirect($no_redirect);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isRedirectDisabled() {
-    return $this->mainFormState->isRedirectDisabled();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setProcessInput($process_input = TRUE) {
-    $this->mainFormState->setProcessInput($process_input);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isProcessingInput() {
-    return $this->mainFormState->isProcessingInput();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setProgrammed($programmed = TRUE) {
-    $this->mainFormState->setProgrammed($programmed);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isProgrammed() {
-    return $this->mainFormState->isProgrammed();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setProgrammedBypassAccessCheck($programmed_bypass_access_check = TRUE) {
-    $this->mainFormState->setProgrammedBypassAccessCheck($programmed_bypass_access_check);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isBypassingProgrammedAccessChecks() {
-    return $this->mainFormState->isBypassingProgrammedAccessChecks();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setRebuildInfo(array $rebuild_info) {
-    $this->mainFormState->setRebuildInfo($rebuild_info);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getRebuildInfo() {
-    return $this->mainFormState->getRebuildInfo();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function addRebuildInfo($property, $value) {
-    $this->mainFormState->addRebuildInfo($property, $value);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setSubmitHandlers(array $submit_handlers) {
-    $this->mainFormState->setSubmitHandlers($submit_handlers);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getSubmitHandlers() {
-    return $this->mainFormState->getSubmitHandlers();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setSubmitted() {
-    $this->mainFormState->setSubmitted();
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isSubmitted() {
-    return $this->mainFormState->isSubmitted();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setTemporary(array $temporary) {
-    $this->mainFormState->setTemporary($temporary);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getTemporary() {
-    return $this->mainFormState->getTemporary();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getTemporaryValue($key) {
-    return $this->mainFormState->getTemporaryValue($key);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setTemporaryValue($key, $value) {
-    $this->mainFormState->setTemporaryValue($key, $value);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function hasTemporaryValue($key) {
-    return $this->mainFormState->getTemporaryValue($key);
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setTriggeringElement($triggering_element) {
-    $this->mainFormState->setTriggeringElement($triggering_element);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function &getTriggeringElement() {
-    return $this->mainFormState->getTriggeringElement();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValidateHandlers(array $validate_handlers) {
-    $this->mainFormState->setValidateHandlers($validate_handlers);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getValidateHandlers() {
-    return $this->mainFormState->getValidateHandlers();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setValidationComplete($validation_complete = TRUE) {
-    $this->mainFormState->setValidationComplete($validation_complete);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function isValidationComplete() {
-    return $this->mainFormState->isValidationComplete();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function getCleanValueKeys() {
-    return $this->mainFormState->getCleanValueKeys();
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function setCleanValueKeys(array $keys) {
-    $this->mainFormState->setCleanValueKeys($keys);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function addCleanValueKey($key) {
-    $this->mainFormState->addCleanValueKey($key);
-    return $this;
-  }
-
-  /**
-   * {@inheritdoc}
-   */
-  public function cleanValues() {
-    $this->mainFormState->cleanValues();
-    return $this;
-  }
-
-}
diff --git a/src/Processor/ProcessorPluginBase.php b/src/Processor/ProcessorPluginBase.php
index 8e9dd3d..4f59473 100644
--- a/src/Processor/ProcessorPluginBase.php
+++ b/src/Processor/ProcessorPluginBase.php
@@ -4,6 +4,7 @@ namespace Drupal\facets\Processor;
 
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Entity\DependencyTrait;
+use Drupal\Core\Form\SubformStateInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\facets\FacetInterface;
 
@@ -31,6 +32,10 @@ class ProcessorPluginBase extends PluginBase implements ProcessorInterface {
    * {@inheritdoc}
    */
   public function submitConfigurationForm(array $form, FormStateInterface $form_state, FacetInterface $facet) {
+    if (!($form_state instanceof SubformStateInterface)) {
+      $trace = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS, 2);
+      trigger_error(sprintf('%s::%s() SHOULD receive %s on line %d, but %s was given. More information is available at https://www.drupal.org/node/2774077.', $trace[1]['class'], $trace[1]['function'], SubformStateInterface::class, $trace[1]['line'], get_class($form_state)), E_USER_DEPRECATED);
+    }
     $this->setConfiguration($form_state->getValues());
   }