fix the case when relation is set to null

src/PersistentData/PersistentDataManager.php

@@ -302,6 +302,8 @@ class PersistentDataManager implements IPersistentDataManager
 
             if ($relationModel !== null) {
                 $model->$methodSet($relationModel->getId());
+            } else {
+                $model->$methodSet(null);
             }
         }
     }