PHP error (array_flip(): Can only flip STRING and INTEGER) after enabling module and going to the config page

Drupal 8.6

Enable module, proceed to > /admin/config/search/splash_redirect/settings

This error appears:

Warning: array_flip(): Can only flip STRING and INTEGER values! in Drupal\Core\Entity\EntityStorageBase->loadMultiple() (line 264 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).
Drupal\Core\Entity\EntityStorageBase->loadMultiple(Array) (Line: 249)
Drupal\Core\Entity\EntityStorageBase->load(NULL) (Line: 527)
Drupal\Core\Entity\Entity::load(NULL) (Line: 44)
Drupal\splash_redirect\Form\SplashRedirectSettingsForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 518)
Drupal\Core\Form\FormBuilder->retrieveForm('splash_redirect_settings_form', Object) (Line: 275)
Drupal\Core\Form\FormBuilder->buildForm('splash_redirect_settings_form', Object) (Line: 93)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
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: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 44)
Drupal\services\StackMiddleware\FormatSetter->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 669)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)