Add a game over check.

At the end of the turn, record all the blocks on the top row. On the next turn, check if any of the blocks on the top row are the same blocks as the one stored. If so, game-over.

Also as part of this task, don't forget to not overwrite any cells on the top row that already has a block on it.

Also recommended: since blocks are all pooled, I highly recommend assigning a unique ID to each and every block.