Sirtuck

Go for it. I haven't worked on this in a long time so I can try to help if you have any questions on it, no promises though.

Okay, so what it sounds like is that an object is being removed then something is trying to check that object which makes Greenfoot error, I will have a look around the code to see why this would be happening. If you can get it to happen again with any more details that would be appreciated. I also am uploading a new version so that mines now properly trigger on the square enemy.