bug #6496 Fix next page url (danut007ro)

This PR was merged into the 4.x branch.

Discussion
----------

Fix next page url

When using `AssociationField` and there are no more pages for a result the `generateUrl()` method will be called on `null`.

This PR adds a check when calling `$nextPageUrl->generateUrl()`.

Commits
-------

ac3ef8b Fix next page url

src/Orm/EntityPaginator.php

@@ -228,7 +228,7 @@ public function getResultsAsJson(): string
             $nextPageUrl->setController($crudControllerFqcn)->setAction($currentRequest->attributes->get(EA::CRUD_ACTION));
         }
 
-        $jsonResult['next_page'] = $nextPageUrl->generateUrl();
+        $jsonResult['next_page'] = $nextPageUrl?->generateUrl();
 
         return json_encode($jsonResult, \JSON_THROW_ON_ERROR);
     }