piston null pointer exception after tnt use.

TNT piston-head combination can crash game.

What I expected to happen was:
A hole create by the TNT.

What actually happened was:
The game creased.(crash1.txt) After the crash an attempt to enter the world fail resulting in another game crash.(crash2.txt) After another attempt entering the world was successful. The TNT did not leave a hole but it did destroy the piston-head. (Only the piston head.)

Steps to Reproduce:
1. get a piston to extend.
2. place a block of TNT 2 blocks away from the piston head
3. power the block of TNT.