I think this has to do with this block.
If I understand this extension correctly, this event fires when the enemy’s health reaches zero, so you can decide what to do with the enemy. However, you are simply deleting a reference to an enemy sprite that was created in the game update loop.
So I am guessing when another piece of code is accessing an enemy sprite somewhere, that maybe that reference no longer exists.
I think what you need to do is this:
You set a reference to the sprite whose status bar has reached zero, and then delete it. Give that a try and let me know how that goes. If you are unsure, you could ask @jwunderl as he made and/or announced the extension: Status Bars extension (beta)