The "" plugin does not exist on config save in PolicyEditForm.php

Problem/Motivation

Drupal\Component\Plugin\Exception\PluginNotFoundException thrown in Drupal\ Core \Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php when saving configuration in PolicyEditForm

The full error:

An unexpected error has occurred. Please try again later.

Drupal\Component\Plugin\Exception\PluginNotFoundException : The "" plugin does not exist. Valid plugin IDs for Drupal\symfony_mailer\Processor\EmailAdjusterManager are: email_plain, email_cc, email_priority, mailer_url_to_absolute, mailer_wrap_and_convert, email_to, email_subject, mailer_default_headers, email_bcc, mailer_hooks, email_body, email_from, email_theme, email_transport, mailer_inline_css, email_skip_sending, email_reply_to in Drupal\ Core \Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php ).
Drupal\Core\Plugin\DefaultPluginManager->getDefinition('') (Line: 16)
Drupal\Core\Plugin\Factory\ContainerFactory->createInstance('', Array) (Line: 83)
Drupal\Component\Plugin\PluginManagerBase->createInstance('', Array) (Line: 17)
Drupal\symfony_mailer\Processor\AdjusterPluginCollection->initializePlugin('') (Line: 80)
Drupal\Component\Plugin\LazyPluginCollection->get('') (Line: 160)
Drupal\Core\Plugin\DefaultLazyPluginCollection->setInstanceConfiguration('', Array) (Line: 135)
Drupal\Core\Plugin\DefaultLazyPluginCollection->setConfiguration(Array) (Line: 26)
Drupal\symfony_mailer\Form\PolicyEditForm->form(Array, Object) (Line: 106)
Drupal\Core\Entity\EntityForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 536)
Drupal\Core\Form\FormBuilder->retrieveForm('mailer_policy_edit_form', Object) (Line: 375)
Drupal\Core\Form\FormBuilder->rebuildForm('mailer_policy_edit_form', Object, Array) (Line: 633)
Drupal\Core\Form\FormBuilder->processForm('mailer_policy_edit_form', Array, Object) (Line: 325)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 580)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 169)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 81)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 44)
Drupal\redirect_after_login\RedirectMiddleware->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 49)
Asm89\Stack\Cors->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 718)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

1. Go to /admin/config/system/mailer and add a new Policy, After selecting Type and adding Sub-Type it will redirect to Policy edit form.
2. Select any element and press "Add Element".
3. Add element details and press Save on the form.

Please see the video

https://drive.google.com/file/d/1qHwrg8Kub8tGWLzb_3y2DPXHBBv9b_-6/view?usp=sharing.

Proposed resolution

I guess that the #empty_value is causing that error since it is producing empty value for configs (but I am not sure, needs to be verified).

$form['add_actions']['add_select'] = [
  '#type' => 'select',
  '#options' => $options,
  '#empty_value' => '',
  '#empty_option' => $this->t('- Select element to add -'),
];