This is a very odd, reproducable bug that seems to have something to do with redstone power updates. I have seen other bug reports that crash with the same error but none show reproduction steps so I will post this one.

Steps to reproduce.

• Create a new superflat 15w46a world
• Place an impulse command block down with space above it and enter the following command:

  summon FallingSand ~ ~.5 ~ {Block:minecraft:activator_rail,DropItem:0,Time:1}

• Power the command block directly by placing a stone button on its side and clicking it.

Result

The game will crash and give the attached crash report. On re-opening the world the activator rail will be placed on top of the command block and appear to be powered.

Things I've tried

• The game does NOT crash if a lever or redstone block is used in place of a button
• The game sometimes crashes if a stone block is placed next to the command block and a button powers the command block indirectly through the stone. I haven't worked out ways to reproduce the crash with this setup but it happened the first time I tried it.
• The game does NOT crash if a fallingsand entity with a Block value of stone is summoned instead of an activator rail.
• Summoning the fallingsand so it lands on top of a stone block elsewhere in the world that is being powered by a stone button at the moment the sand lands also crashes the game

On a side note, the activator rail appears to remain on once the power has been removed until it receives a block update.