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.

Yeah. First off instead of using getX() and getY() I store the x and y values in doubles, this helps maintain accuracy. So once the user clicks I grab the mouseInfo. Using this I get an xDis and yDis. From these I find the angle from the player to the mouse position. Now with the angle I use basic trig to find the x portion of the angle and the y portion of the angle. Using these I multiply them times the speed of my object and add them to the x or y storage values I keep. Then set the location of the object to x, y. That is the basic setup, if you have any other questions or need clarification please let me know.