CRAFTBOOK-2445 - Reported by noneandnonly

I just updated from #1839 (it was I think) to #1844, my server was running perfectly with CraftBook for about a month now, but as I updated this error started to come up and crashed my server: http://pastebin.com/3Sq2EBNc
Please fix this as soon as possible. Was also happening in #1848.

Comment by me4502

Erm.. That looks like it tried to unload something that doesn't exist.. So maybe some chunk issue? But I'll check the code anyway