Vault dimension creation problem

[simon987]

I assume that it happens when lastLayerWorlds.size() is 0

Stack trace:

Exception in thread "Thread-10" java.lang.IllegalArgumentException: bound must be positive
	at java.util.Random.nextInt(Random.java:388)
	at net.simon987.npcplugin.VaultDimension.<init>(VaultDimension.java:140)
	at net.simon987.npcplugin.VaultDoor.initialize(VaultDoor.java:159)
	at net.simon987.npcplugin.event.WorldCreationListener.handle(WorldCreationListener.java:136)
	at net.simon987.server.event.GameEventDispatcher.dispatch(GameEventDispatcher.java:33)
	at net.simon987.server.game.world.WorldGenerator.generateWorld(WorldGenerator.java:188)
	at net.simon987.server.game.GameUniverse.createWorld(GameUniverse.java:167)
	at net.simon987.server.game.GameUniverse.getWorld(GameUniverse.java:109)
	at net.simon987.server.game.objects.GameObject.incrementLocation(GameObject.java:66)
	at net.simon987.npcplugin.NonPlayerCharacter.gotoWorld(NonPlayerCharacter.java:154)
	at net.simon987.npcplugin.HarvestTask.tick(HarvestTask.java:98)
	at net.simon987.npcplugin.HarvesterNPC.update(HarvesterNPC.java:43)
	at net.simon987.server.game.world.World.update(World.java:183)
	at net.simon987.server.GameServer.tick(GameServer.java:192)
	at net.simon987.server.GameServer.run(GameServer.java:144)
	at java.lang.Thread.run(Thread.java:748)