I have a feeling this is something to do with it :/
java.lang.NullPointerException
at greenfoot.World.addObject(World.java:392)
at GameOver.prepare(GameOver.java:67)
at GameOver.<init>(GameOver.java:29)
at Zombie.act(Zombie.java:43)
at greenfoot.core.Simulation.actActor(Simulation.java:568)
at greenfoot.core.Simulation.runOneLoop(Simulation.java:526)
at greenfoot.core.Simulation.runContent(Simulation.java:215)
at greenfoot.core.Simulation.run(Simulation.java:205)