Thanks for trying it. That's an interesting bug. I'm not sure how your current monster got set to None, maybe it's an issue where I forgot to set them to rez after you lose and then when it starts a battle in that state, it looks for someone to put in first but can't find them... I've written a lot of patches since the 7drl release but I'll double check that the bug doesn't still exist.
Edit: I was wrong, it was a much less complex bug where the whole "if self.opp_tag:" block in battle.py tries to tag in the player's monster instead, but none is specified. There's such a lot chance of opp tagging that it never came up in testing. (Which is why I should write unit tests I guess...)