only allow TitleResolver to check for alternative field on fieldable entities. Fixes #3253835

src/TitleResolver.php

@@ -7,6 +7,7 @@ use Drupal\Core\Url;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\Routing\Route;
 use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Entity\FieldableEntityInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Component\Utility\Xss;
@@ -61,7 +62,7 @@ class TitleResolver extends ControllerTitleResolver {
     }
     if ($entity !== NULL) {
       $alternative_title_field = $this->config->get(EasyBreadcrumbConstants::ALTERNATIVE_TITLE_FIELD);
-      if ($entity->hasField($alternative_title_field) && !$entity->get($alternative_title_field)
+      if ($entity instanceof FieldableEntityInterface && $entity->hasField($alternative_title_field) && !$entity->get($alternative_title_field)
         ->isEmpty()) {
         return Xss::filter($entity->get($alternative_title_field)->value);
       }