Making multiple duplicates of Blend Trees and deleting them gives errors

1. Create a Blend Tree and duplicate it,
2. Mark a few of the duplicated Blend Trees and duplicate those
3. Make between 20-30 blend trees this way
4. Mark all of them but 3-4 and delete
5. Delete the remaining 3-4
6. Error appears in the console:
MissingReferenceException: The object of type 'AnimatorState' has been destroyed but you are still trying to access it.