http://pastebin.com/recfpJka

I had just created a new world and was hitting a chicken with a wood axe.

http://pastebin.com/L5yv9GL2

Reproduced here while killing other mobs.

Oh yeah, that's more or less a beginners mistake I managed to make there. The reason you're crashing is because of a missing null check (hence the NPE). For now I would advice you downgrade to 1.9.1.1 until I have something out that corrects this error.
The null check I though about was indeed present, so this is something else.

I was able to reproduce it; went on a rampage against a bunch of chickens, swinging a wooden axe all over the place. I do believe I've found a solution, but I might conduct some more testing before reaching that conclusion.