Oh sorry for the late reply I forgot to set notify on this topic so I only found your reply now when checking if there is anything.
It can be set to "None", than it ignores and triggers for everything.
But it also can be set to NULL
(nothing is displayed there, no tag and not even "None")
In this state it does not do anything and there is no error message, so this is hard to find.
Br,
Peter