prevent php-warnings in courseware, fixes #4664

Closes #4664

Merge request studip/studip!3474

lib/classes/JsonApi/Routes/Courseware/Rel/BlocksEditBlocker.php

@@ -39,7 +39,7 @@ class BlocksEditBlocker extends RelationshipsController
             }
         }
 
-        $related->edit_blocker_id = $editor->id;
+        $related->edit_blocker_id = $editor->id ?? null;
         $related->store();
 
         return $this->getCodeResponse(204);

lib/classes/JsonApi/Routes/Courseware/Rel/ContainersEditBlocker.php

@@ -39,7 +39,7 @@ class ContainersEditBlocker extends RelationshipsController
             }
         }
 
-        $related->edit_blocker_id = $editor->id;
+        $related->edit_blocker_id = $editor->id ?? null;
         $related->store();
 
         return $this->getCodeResponse(204);

lib/classes/JsonApi/Routes/Courseware/Rel/StructuralElementsEditBlocker.php

@@ -39,7 +39,7 @@ class StructuralElementsEditBlocker extends RelationshipsController
             }
         }
 
-        $related->edit_blocker_id = $editor->id;
+        $related->edit_blocker_id = $editor->id ?? null;
         $related->store();
 
         return $this->getCodeResponse(204);