I've looked through your source and I cannot seem to identify the problem. You can see and figure out how to recreate the problem at:

http://stackoverflow.com/questions/18625659/bukkit-server-error?noredirect=1#comment27422933_18625659

I tried to solve it but I can't seem to find what's wrong and I don't have time to dig into the code right now. Could you help this person out?

I have responded to the user's request, the issue should be resolved. Thanks! =)