What were you doing when the issue happened?

Trying to swap items positions using number keys while bag was open. But I clicked the bag number position by mistake.

What did you expect to happen?

Swap items position

What happened instead?

Crash

Crashlog on pastebin/gist

http://pastebin.com/QvgpfZwX

Forge and Minecraft version?

forge-12.18.1.2088

Any other known modifications to Minecraft/Forge itself that are known to cause issues?

Any other info which might help.

Thanks for reporting this. 3.0.4c is about to be released on Curseforge and should fix the issue, let me know if it doesn't seem to have worked :)

No problem, I've only seen it happen once, just thought it would be nice to report :)