Fix custom event owner predicate

aleksis/core/util/predicates.py

@@ -177,4 +177,4 @@ def is_own_celery_task(user: User, obj: Model) -> bool:
 @predicate
 def is_custom_event_owner(user: User, custom_event: CustomEvent) -> bool:
     """Predicate which checks if the user is the owner of the provided custom event."""
-    return user.person == group.owner
+    return user.person == custom_event.owner