The database connection is not serializable.

Problem/Motivation

An AJAX HTTP error occurred.
HTTP Result Code: 500
Debugging information follows.
Path: /en/admin/structure/types/manage/article/add-field/node/field_type_of_content_ref?destinations%5B0%5D%5Broute_name%5D=entity.node.field_ui_fields&destinations%5B0%5D%5Broute_parameters%5D%5Bentity_type%5D=node&destinations%5B0%5D%5Broute_parameters%5D%5Bfield_name%5D=field_type_of_content_ref&destinations%5B0%5D%5Broute_parameters%5D%5Bnode_type%5D=article&destinations%5B1%5D=/en/admin/structure/types/manage/article/fields/add-field
StatusText: error
ResponseText: The website encountered an unexpected error. Try again later.LogicException: The database connection is not serializable. This probably means you are serializing an object that has an indirect reference to the database connection. Adjust your code so that is not necessary. Alternatively, look at DependencySerializationTrait as a temporary solution. in Drupal\Core\Database\Connection->__sleep() (line 1920 of core/lib/Drupal/Core/Database/Connection.php). serialize() (Line: 14)
Drupal\Component\Serialization\PhpSerialize::encode() (Line: 129)
Drupal\Core\KeyValueStore\DatabaseStorageExpirable->doSetWithExpire() (Line: 145)
Drupal\Core\KeyValueStore\DatabaseStorageExpirable->setWithExpire() (Line: 193)
Drupal\Core\Form\FormCache->setCache() (Line: 463)
Drupal\Core\Form\FormBuilder->setCache() (Line: 441)
Drupal\Core\Form\FormBuilder->rebuildForm() (Line: 633)
Drupal\Core\Form\FormBuilder->processForm() (Line: 326)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 48)
Drupal\Core\Entity\EntityFormBuilder->getForm() (Line: 62)
Drupal\field_ui\Controller\FieldConfigAddController->fieldConfigAddConfigureForm()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 638)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 181)
Symfony\Component\HttpKernel\HttpKernel->handleRaw() (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle() (Line: 54)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle() (Line: 53)
Drupal\Core\StackMiddleware\Session->handle() (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle() (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle() (Line: 116)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 90)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 206)
Drupal\cleantalk\EventSubscriber\BootSubscriber->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 36)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 51)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 741)
Drupal\Core\DrupalKernel->handle() (Line: 19)

Steps to reproduce

Install the module and create a new entity reference field