TriggerMontageEndedEvent is calling RemoveAtSwap after the NotifyEnd callback which is not safe if the ActiveAninNotifyState array is modified by the callback.
WorkAround: Add a Delay node before calling DestroyActor. Licensee suggests "It might be safer to cache the array before iterating or somehow lock the array or block certain functions."
*Project has been attached for easier replication
Result: Editor crash.
Expected: Montage to play successfully, calling destroy actor at the notify end function.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-74191 in the post.