The wither chest itself seems to be properly functional, although I don't know what is meant to happen when you break it. (It just destroys its contents)
The top of the chest, however, allows you to break it instantly without any tool.
When broken it also breaks, and drops, the bottom half (instantly destroying all the loot :c ) and more importantly drops the top half.

This top half is invisible on the ground and when picked up - but trying to mouse over it in your inventory will crash the game immediately.

• Crash Report Gist

PS: Thanks for such a cool mod! Liking it very much so far.

Thanks, I'll look into this.

Fixed these issues for the next release (beta0.9.0)