Fatal error deleting host entity when wait list enabled

Problem/Motivation

I got this error when I tried to delete a node that had registrations on it:

The website encountered an unexpected error. Try again later.

Error: Call to a member function getSetting() on null in Drupal\registration_waitlist\EventSubscriber\RegistrationEventSubscriber->onDelete() (line 80 of modules/contrib/registration/modules/registration_waitlist/src/EventSubscriber/RegistrationEventSubscriber.php).
Symfony\Component\EventDispatcher\EventDispatcher::Symfony\Component\EventDispatcher\{closure}(Object, 'registration.registration.delete', Object) (Line: 206)
Symfony\Component\EventDispatcher\EventDispatcher->callListeners(Array, 'registration.registration.delete', Object) (Line: 56)
Symfony\Component\EventDispatcher\EventDispatcher->dispatch(Object, 'registration.registration.delete') (Line: 57)
Drupal\registration\RegistrationStorage->invokeHook('delete', Object) (Line: 501)
Drupal\Core\Entity\EntityStorageBase->delete(Array) (Line: 753)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->delete(Array) (Line: 132)
Drupal\registration_purger\RegistrationPurger->purgeRegistrations(Object) (Line: 75)
Drupal\registration_purger\RegistrationPurger->onEntityDelete(Object) (Line: 14)
registration_purger_entity_delete(Object)
call_user_func_array('registration_purger_entity_delete', Array) (Line: 389)
Drupal\Core\Extension\ModuleHandler->Drupal\Core\Extension\{closure}('registration_purger_entity_delete', 'registration_purger') (Line: 340)
Drupal\Core\Extension\ModuleHandler->invokeAllWith('entity_delete', Object) (Line: 388)
Drupal\Core\Extension\ModuleHandler->invokeAll('entity_delete', Array) (Line: 221)
Drupal\Core\Entity\EntityStorageBase->invokeHook('delete', Object) (Line: 1038)
Drupal\Core\Entity\ContentEntityStorageBase->invokeHook('delete', Object) (Line: 501)
Drupal\Core\Entity\EntityStorageBase->delete(Array) (Line: 753)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->delete(Array) (Line: 378)
Drupal\Core\Entity\EntityBase->delete() (Line: 71)
Drupal\Core\Entity\ContentEntityDeleteForm->submitForm(Array, Object) (Line: 108)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 45)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 615)
Drupal\Core\Form\FormBuilder->processForm('node_event_delete_form', Array, Object) (Line: 347)
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: 634)
Drupal\Core\Render\Renderer::Drupal\Core\Render\{closure}()
Fiber->resume() (Line: 649)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 183)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 76)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 53)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 28)
Drupal\Core\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle(Object, 1, 1) (Line: 118)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 92)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 50)
Drupal\ban\BanMiddleware->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: 53)
Drupal\Core\StackMiddleware\AjaxPageState->handle(Object, 1, 1) (Line: 54)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle(Object, 1, 1) (Line: 745)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Drupal 11.3.2

PHP 8.3

Install registration, registration_waitlist, and registration_purger

Might need to have some registrations on the node while trying to delete it; I don't think I got any error when I tried deleting an event node earlier that did not have any registrations on it.

Proposed resolution

I just encountered this a few minutes ago and haven't done deep tests, but my guess at this point is that what is happening is this order of operations:

1. Node gets deleted.
2. Registrations start being purged.
3. Once one registration is deleted, that means there's room to move another from the waitlist to complete and it tries to do so.
4. Except that the node it is trying to use that for, the $host_entity at the identified line 80, no longer exists.

If that's true, that solution might just be a type check to confirm that $host_entity is an instance of HostEntityInterface, not null, before proceeding to get its settings.

If I find some time, I'll come back and see what I can do for testing a solution, but in the meantime I'm happy for those more knowledgeable to run with it.