Game crash

7x7 Grid of pistons facing down, blocks and redstone on top of it causes the game to crash when activated 2nd time.First time redstone on top of pistons pops when activated and after replacing the redstone and activating it again game crashes. (When pistons retract)

http://i.imgur.com/jiNdo9F.gifv

Edit: Report file http://pastebin.com/raw/1jgrmK7p