Failing to kick in saving a D11.3.12 Article node

I am testing v1.5.0 with Drupal 11.

Reproducing

1) Module is enabled.
2) Check and make a small tweak to the wording of the module's warning message.
3) Attempt to save a new Article with exactly the same node title as an existing one.

This results in…

Warning: Trying to access array offset on null in unique_alias_checker_validate() (line 53 of modules\contrib\unique_alias_checker\unique_alias_checker.module).
unique_alias_checker_validate() (Line: 87)
Drupal\Core\Form\FormValidator->executeValidateHandlers() (Line: 280)
Drupal\Core\Form\FormValidator->doValidateForm() (Line: 123)
Drupal\Core\Form\FormValidator->validateForm() (Line: 611)
Drupal\Core\Form\FormBuilder->processForm() (Line: 347)
Drupal\Core\Form\FormBuilder->buildForm() (Line: 73)
Drupal\Core\Controller\FormController->getContentResult()
call_user_func_array() (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 634)
Drupal\Core\Render\Renderer::Drupal\Core\Render\{closure}()
Fiber->start() (Line: 635)
Drupal\Core\Render\Renderer->executeInRenderContext() (Line: 121)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext() (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 183)
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: 32)
Drupal\big_pipe\StackMiddleware\ContentLength->handle() (Line: 118)
Drupal\page_cache\StackMiddleware\PageCache->pass() (Line: 92)
Drupal\page_cache\StackMiddleware\PageCache->handle() (Line: 45)
Drupal\markdownify\StackMiddleware\MarkdownifyNegotiationMiddleware->handle() (Line: 62)
Drupal\crowdsec\Middleware->handle() (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle() (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle() (Line: 53)
Drupal\Core\StackMiddleware\AjaxPageState->handle() (Line: 54)
Drupal\Core\StackMiddleware\StackedHttpKernel->handle() (Line: 745)
Drupal\Core\DrupalKernel->handle() (Line: 19)

The above warning message occurs twice and then the Article gets saved using a 'dodgy' alias…

/articles/supporting-computer-science-pgceite-1

It may be relevant to point out that I am also using the 'Add another' module as evidenced by the post-save message (line 2)…

System under test

Drupal 11.3.12
A custom subtheme Bootstrap5 (Bootstrap5 v4.05)
Apache/2.4.62 (Win64) OpenSSL/3.0.15 mod_fcgid/2.3.9
PHP 8.3.26
PHP Memory limit: 512M
MySQL 8.4.3
PHP Caching module: OpCache