Simple Space Game

java.lang.IllegalStateException: Actor not in world. An attempt was made to use the actor's location while it is not in the world. Either it has not yet been inserted, or it has been removed. at greenfoot.Actor.failIfNotInWorld(Actor.java:656) at greenfoot.Actor.getOneIntersectingObject(Actor.java:905) at Objects.onContact(Objects.java:33) at Astroid.removeAstroid(Astroid.java:56) at Astroid.act(Astroid.java:19) at greenfoot.core.Simulation.actActor(Simulation.java:507) at greenfoot.core.Simulation.runOneLoop(Simulation.java:470) at greenfoot.core.Simulation.runContent(Simulation.java:204) at greenfoot.core.Simulation.run(Simulation.java:194)

Looks like you're removing the actor, and then checking for collision (which requires that the actor is still in the world). Change the order around so you check for collision first. Alternatively, search for the exception in the discussion forum - this is a common problem.