diff --git a/src/Form/XmlSitemapEntitiesSettingsForm.php b/src/Form/XmlSitemapEntitiesSettingsForm.php
index 17a9508..cafd231 100644
--- a/src/Form/XmlSitemapEntitiesSettingsForm.php
+++ b/src/Form/XmlSitemapEntitiesSettingsForm.php
@@ -134,12 +136,13 @@ class XmlSitemapEntitiesSettingsForm extends ConfigFormBase implements Container
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
     $bundles = $this->entityManager->getAllBundleInfo();
-    $entity_values = $form_state['values']['entity_types'];
+    $entity_values = $form_state->getValue('entity_types');
     $config = $this->config('xmlsitemap.settings');
+    $settings = $form_state->getValue('settings');
     foreach ($entity_values as $key => $value) {
       if ($value) {
         foreach ($bundles[$key] as $bundle_key => $bundle_value) {
-          if (!$form_state['values']['settings'][$key][$bundle_key]['settings']['bundle']) {
+          if (!$settings[$key][$bundle_key]['settings']['bundle']) {
             xmlsitemap_link_bundle_delete($key, $bundle_key, TRUE);
           }
           else {
diff --git a/src/Form/XmlSitemapForm.php b/src/Form/XmlSitemapForm.php
index c8a3554..24d6d39 100644
--- a/src/Form/XmlSitemapForm.php
+++ b/src/Form/XmlSitemapForm.php
@@ -59,15 +59,18 @@ class XmlSitemapForm extends EntityForm {
    * {@inheritdoc}
    */
   public function save(array $form, FormStateInterface $form_state) {
-    if (!isset($form_state['values']['context'])) {
-      $form_state['values']['context'] = xmlsitemap_get_current_context();
+    if (!$form_state->hasValue('context')) {
+      $form_state->setValue('context', xmlsitemap_get_current_context());
     }
-    if (isset($form_state['values']['context']['language']) && $form_state['values']['context']['language'] == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
-      unset($form_state['values']['context']['language']);
+    if ($form_state->hasValue(['context', 'language'])) {
+      $language = $form_state->getValue(['context', 'language']);
+      if ($language == LanguageInterface::LANGCODE_NOT_SPECIFIED) {
+        $form_state->unsetValue(['context', 'language']);
+      }
     }
-    $this->entity->context = $form_state['values']['context'];
-    $context = $form_state['values']['context'];
-    $this->entity->label = $form_state['values']['label'];
+    $context = $form_state->getValue('context');
+    $this->entity->context = $context;
+    $this->entity->label = $form_state->getValue('label');
     $this->entity->id = xmlsitemap_sitemap_get_context_hash($context);
 
     try {
diff --git a/src/Form/XmlSitemapLinkBundleSettingsForm.php b/src/Form/XmlSitemapLinkBundleSettingsForm.php
index 6835264..b9a701a 100644
--- a/src/Form/XmlSitemapLinkBundleSettingsForm.php
+++ b/src/Form/XmlSitemapLinkBundleSettingsForm.php
@@ -112,14 +113,15 @@ class XmlSitemapLinkBundleSettingsForm extends ConfigFormBase implements Contain
       $entity_info = $form['xmlsitemap']['#entity_info'];
       if (isset($entity_info['bundle keys']['bundle'])) {
         $bundle_key = $entity_info['bundle keys']['bundle'];
-        if (isset($form_state['values'][$bundle_key])) {
-          $bundle = $form_state['values'][$bundle_key];
+        if ($form_state->hasValue($bundle_key)) {
+          $bundle = $form_state->getValue($bundle_key);
           $form['xmlsitemap']['#bundle'] = $bundle;
         }
       }
     }
 
-    xmlsitemap_link_bundle_settings_save($this->entity_type, $this->bundle_type, $form_state['values']['xmlsitemap'], TRUE);
+    $xmlsitemap = $form_state->getValue('xmlsitemap');
+    xmlsitemap_link_bundle_settings_save($this->entity_type, $this->bundle_type, $xmlsitemap, TRUE);
 
     $entity_info = $form['xmlsitemap']['#entity_info'];
     if (!empty($form['xmlsitemap']['#show_message'])) {
@@ -127,9 +129,10 @@ class XmlSitemapLinkBundleSettingsForm extends ConfigFormBase implements Contain
     }
 
     // Unset the form values since we have already saved the bundle settings and
-    // we don't want these values to get saved as variables in-case this form
-    // also uses system_settings_form().
-    unset($form_state['values']['xmlsitemap']);
+    // we don't want these values to get saved as configuration, depending on how
+    // the form saves the form values.
+    $form_state->unsetValue('xmlsitemap');
+
     parent::submitForm($form, $form_state);
   }
 
diff --git a/src/Form/XmlSitemapRebuildForm.php b/src/Form/XmlSitemapRebuildForm.php
index 21828a6..614f8c7 100644
--- a/src/Form/XmlSitemapRebuildForm.php
+++ b/src/Form/XmlSitemapRebuildForm.php
@@ -95,7 +96,9 @@ class XmlSitemapRebuildForm extends ConfigFormBase {
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
     // Save any changes to the frontpage link.
-    $batch = xmlsitemap_rebuild_batch($form_state['values']['entities'], $form_state['values']['save_custom']);
+    $entities = $form_state->getValue('entities');
+    $save_custom = $form_state->getValue('save_custom');
+    $batch = xmlsitemap_rebuild_batch($entities, $save_custom);
     batch_set($batch);
 
     $form_state->setRedirect('xmlsitemap.admin_search');
diff --git a/src/Form/XmlSitemapSettingsForm.php b/src/Form/XmlSitemapSettingsForm.php
index 412900d..00ad1f2 100644
--- a/src/Form/XmlSitemapSettingsForm.php
+++ b/src/Form/XmlSitemapSettingsForm.php
@@ -217,13 +217,14 @@ class XmlSitemapSettingsForm extends ConfigFormBase {
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
     // Check that the chunk size will not create more than 1000 chunks.
-    $chunk_size = $form_state['values']['chunk_size'];
+    $chunk_size = $form_state->getValue('chunk_size');
     if ($chunk_size != 'auto' && $chunk_size != 50000 && (xmlsitemap_get_link_count() / $chunk_size) > 1000) {
       $form_state->setErrorByName('chunk_size', t('The sitemap page link count of @size will create more than 1,000 sitemap pages. Please increase the link count.', array('@size' => $chunk_size)));
     }
 
-    $base_url = &$form_state['values']['xmlsitemap_base_url'];
+    $base_url = $form_state->getValue('xmlsitemap_base_url');
     $base_url = rtrim($base_url, '/');
+    $form_state->setValue('xmlsitemap_base_url', $base_url);
     if ($base_url != '' && !UrlHelper::isValid($base_url, TRUE)) {
       $form_state->setErrorByName('xmlsitemap_base_url', t('Invalid base URL.'));
     }
@@ -237,7 +238,7 @@ class XmlSitemapSettingsForm extends ConfigFormBase {
   public function submitForm(array &$form, FormStateInterface $form_state) {
     // Save any changes to the frontpage link.
     $config = $this->config('xmlsitemap.settings');
-    $values = $form_state['values'];
+    $values = $form_state->getValues();
     if (isset($form['frontpage'])) {
       $frontpage_priority = $values['xmlsitemap_frontpage_priority'];
       $frontpage_changefreq = $values['xmlsitemap_frontpage_changefreq'];
diff --git a/xmlsitemap.module b/xmlsitemap.module
index d40c5f9..6aa95d7 100644
--- a/xmlsitemap.module
+++ b/xmlsitemap.module
@@ -1069,14 +1069,15 @@ function xmlsitemap_calculate_changefreq($timestamps) {
  * calls to variable_set().
  */
 function xmlsitemap_form_submit_flag_regenerate(array $form, FormStateInterface $form_state) {
-  foreach ($form_state['values'] as $variable => $value) {
+  $values = $form_state->getValues();
+  foreach ($values as $variable => $value) {
     if (\Drupal::config('xmlsitemap.settings')->get($variable) == NULL) {
       $stored_value = 'not_a_variable';
     }
     else {
       $stored_value = \Drupal::config('xmlsitemap.settings')->get($variable);
     }
-    if (is_array($value) && !empty($form_state['values']['array_filter'])) {
+    if (is_array($value) && !$form_state->isValueEmpty('array_filter')) {
       $value = array_keys(array_filter($value));
     }
     if ($stored_value != 'not_a_variable' && $stored_value != $value) {
@@ -1225,7 +1227,7 @@ function xmlsitemap_add_form_link_options(array &$form, $entity, $bundle, $id) {
  * Submit callback for the entity form to save.
  */
 function xmlsitemap_process_form_link_options(array $form, FormStateInterface $form_state) {
-  $link = &$form_state['values']['xmlsitemap'];
+  $link = $form_state->getValue('xmlsitemap');
   $fields = array('status' => XMLSITEMAP_STATUS_DEFAULT, 'priority' => XMLSITEMAP_PRIORITY_DEFAULT);
 
   foreach ($fields as $field => $default) {
@@ -1237,9 +1239,10 @@ function xmlsitemap_process_form_link_options(array $form, FormStateInterface $f
       $link[$field . '_override'] = 1;
     }
   }
+  $form_state->setValue('xmlsitemap', $link);
 
-  $entity = $form_state['controller']->getEntity();
-  $entity->xmlsitemap = $form_state['values']['xmlsitemap'];
+  $entity = $form_state->getFormObject()->getEntity();
+  $entity->xmlsitemap = $form_state->getValue('xmlsitemap');
 }
 
 /**
@@ -1255,14 +1258,15 @@ function xmlsitemap_link_bundle_settings_form_submit($form, &$form_state) {
     $entity_info = $form['xmlsitemap']['#entity_info'];
     if (isset($entity_info['bundle keys']['bundle'])) {
       $bundle_key = $entity_info['bundle keys']['bundle'];
-      if (isset($form_state['values'][$bundle_key])) {
-        $bundle = $form_state['values'][$bundle_key];
+      if ($form_state->hasValue($bundle_key)) {
+        $bundle = $form_state->getValue($bundle_key);
         $form['xmlsitemap']['#bundle'] = $bundle;
       }
     }
   }
 
-  xmlsitemap_link_bundle_settings_save($entity, $bundle, $form_state['values']['xmlsitemap']);
+  $xmlsitemap = $form_state->getValue('xmlsitemap');
+  xmlsitemap_link_bundle_settings_save($entity, $bundle, $xmlsitemap);
 
   $entity_info = $form['xmlsitemap']['#entity_info'];
   if (!empty($form['xmlsitemap']['#show_message'])) {
@@ -1270,9 +1274,9 @@ function xmlsitemap_link_bundle_settings_form_submit($form, &$form_state) {
   }
 
   // Unset the form values since we have already saved the bundle settings and
-  // we don't want these values to get saved as variables in-case this form
-  // also uses system_settings_form().
-  unset($form_state['values']['xmlsitemap']);
+  // we don't want these values to get saved as configuration, depending on how
+  // the form saves the form values.
+  $form_state->unsetValue('xmlsitemap');
 }
 
 /**
diff --git a/xmlsitemap_custom/src/Form/XmlSitemapCustomAddForm.php b/xmlsitemap_custom/src/Form/XmlSitemapCustomAddForm.php
index 1e18d51..8a6fccf 100644
--- a/xmlsitemap_custom/src/Form/XmlSitemapCustomAddForm.php
+++ b/xmlsitemap_custom/src/Form/XmlSitemapCustomAddForm.php
@@ -160,11 +162,12 @@ class XmlSitemapCustomAddForm extends ConfigFormBase {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
-    $link = &$form_state['values'];
+    $link = $form_state->getValues();
 
     // Make sure we trim and normalize the path first.
     $link['loc'] = trim($link['loc']);
     $link['loc'] = $this->aliasManager->getPathByAlias($link['loc'], $link['language']);
+    $form_state->setValue('loc', $link['loc']);
     $query = db_select('xmlsitemap');
     $query->fields('xmlsitemap');
     $query->condition('type', 'custom');
@@ -191,7 +194,7 @@ class XmlSitemapCustomAddForm extends ConfigFormBase {
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    $link = $form_state['values'];
+    $link = $form_state->getValues();
     $this->linkStorage->save($link);
     drupal_set_message(t('The custom link for %loc was saved.', array('%loc' => $link['loc'])));
 
diff --git a/xmlsitemap_custom/src/Form/XmlSitemapCustomEditForm.php b/xmlsitemap_custom/src/Form/XmlSitemapCustomEditForm.php
index 5ad8942..da1c954 100644
--- a/xmlsitemap_custom/src/Form/XmlSitemapCustomEditForm.php
+++ b/xmlsitemap_custom/src/Form/XmlSitemapCustomEditForm.php
@@ -181,9 +181,10 @@ class XmlSitemapCustomEditForm extends ConfigFormBase {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
-    $link = &$form_state['values'];
+    $link = $form_state->getValues();
     $link['loc'] = trim($link['loc']);
     $link['loc'] = $this->aliasManager->getPathByAlias($link['loc'], $link['language']);
+    $form_state->setValue('loc', $link['loc']);
     try {
       $client = new Client();
       $res = $client->get(Url::fromRoute('<front>', [], array('absolute' => TRUE)) . $link['loc']);
@@ -196,7 +199,7 @@ class XmlSitemapCustomEditForm extends ConfigFormBase {
    * {@inheritdoc}
    */
   public function submitForm(array &$form, FormStateInterface $form_state) {
-    $link = $form_state['values'];
+    $link = $form_state->getValues();
     $this->linkStorage->save($link);
     drupal_set_message(t('The custom link for %loc was saved.', array('%loc' => $link['loc'])));
 
diff --git a/xmlsitemap_engines/src/Form/XmlSitemapEnginesSettingsForm.php b/xmlsitemap_engines/src/Form/XmlSitemapEnginesSettingsForm.php
index f97d467..943df6d 100644
--- a/xmlsitemap_engines/src/Form/XmlSitemapEnginesSettingsForm.php
+++ b/xmlsitemap_engines/src/Form/XmlSitemapEnginesSettingsForm.php
@@ -119,14 +119,16 @@ class XmlSitemapEnginesSettingsForm extends ConfigFormBase {
    * {@inheritdoc}
    */
   public function validateForm(array &$form, FormStateInterface $form_state) {
-    $custom_urls = preg_split('/[\r\n]+/', $form_state['values']['custom_urls'], -1, PREG_SPLIT_NO_EMPTY);
+    $custom_urls = $form_state->getValue('custom_urls');
+    $custom_urls = preg_split('/[\r\n]+/', $custom_urls, -1, PREG_SPLIT_NO_EMPTY);
     foreach ($custom_urls as $custom_url) {
       $url = xmlsitemap_engines_prepare_url($custom_url, '');
       if (!UrlHelper::isValid($url, TRUE)) {
         $form_state->setErrorByName($custom_url, t('Invalid URL %url.', array('%url' => $custom_url)));
       }
     }
-    $form_state['values']['custom_urls'] = implode("\n", $custom_urls);
+    $custom_urls = implode("\n", $custom_urls);
+    $form_state->setValue('custom_urls', $custom_ruls);
     parent::validateForm($form, $form_state);
   }
 
@@ -142,7 +144,7 @@ class XmlSitemapEnginesSettingsForm extends ConfigFormBase {
       'xmlsitemap_engines_submit_updated',
       'custom_urls'
     );
-    $values = $form_state['values'];
+    $values = $form_state->getValues();
     foreach ($keys as $key) {
       if (isset($state_variables[$key])) {
         $this->state->set($key, $values[$key]);