Stacktrace: https://gist.github.com/shadowfacts/f781f15167bcb17e409b

After the exception, the client freezes and according to the log, the integrated server sees that the player has lost connection, the world saves, however the game does not actually crash. The game freezes and must manually be killed.

Yep I'm getting this too.

What did you do to trigger this?

I middle-clicked in my player's inventory.

Yes, but that shouldn't happen. Does it reproduce? Or was it one off? Try with latest 1.8.9 or 1.9 build please.

I tried to reproduce this a number of times with the 1.9 version and I couldn't reproduce it. This definitely happened multiple times before. It appears to have been fixed somewhere in the 1.9 update.

Thanks for checking!