Saving registration settings fails when the conflict module is installed

Problem/Motivation

Saving a registration_settings entity can fatal in the registration_waitlist submodule when the capacity is increased. During the registration_settings update event, the waitlist event subscriber assumes a host entity exists on the RegistrationSettings object and calls getSetting() on null, causing a fatal error.

This blocks registration owners from updating registration settings and results in the generic “unexpected error” message on save.

This only occurs when the conflict module is installed and enabled.

Steps to reproduce

1. Enable the registration and registration_waitlist modules.
2. Create a host entity configured for registration (for example, an Event node using a registration type).
3. Open its registration settings, set capacity to a non-zero value, and save.
4. Edit the existing registration settings entity, increase capacity, and save.
5. Observe a fatal error: Call to a member function getSetting() on null in RegistrationSettingsEventSubscriber::onSettingsUpdate() (line 64).

Error shown on save:

The website encountered an unexpected error. Try again later.

Error: Call to a member function getSetting() on null in Drupal\registration_waitlist\EventSubscriber\RegistrationSettingsEventSubscriber->onSettingsUpdate() (line 64 of modules/contrib/registration/modules/registration_waitlist/src/EventSubscriber/RegistrationSettingsEventSubscriber.php).

call_user_func() (Line: 111)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch() (Line: 57)
Drupal\registration\RegistrationStorage->invokeHook() (Line: 564)
Drupal\Core\Entity\EntityStorageBase->doPostSave() (Line: 781)
Drupal\Core\Entity\ContentEntityStorageBase->doPostSave() (Line: 489)
Drupal\Core\Entity\EntityStorageBase->save() (Line: 806)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save() (Line: 354)
Drupal\Core\Entity\EntityBase->save() (Line: 293)
Drupal\Core\Entity\EntityForm->save() (Line: 63)
Drupal\registration\Form\RegistrationSettingsForm->save()
call_user_func_array() (Line: 129)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers() (Line: 67)
Drupal\Core\Form\FormSubmitter->doSubmitForm() (Line: 597)
Drupal\Core\Form\FormBuilder->processForm() (Line: 326)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult() (Line: 39)
Drupal\layout_builder\Controller\LayoutBuilderHtmlEntityFormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 637)
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: 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: 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)

Proposed resolution

Fix the getValue method of the HostEntityItem class to work properly. This method is called by the conflict module. The method is not used internally, so the problem has been hidden.

Add tests to ensure the problem stays fixed.