I peeled apart a save file, and this seems to happen if you save when you are below ground or if you were very recently below ground. The recommendation of wandering around in the (ground floor) wilderness for a large chunk of turns is a good one as it clears out any improperly saved below-ground locations. From what I can guess, the error "NullReferenceException: Object reference not set to an instance of an object" is because the game is trying to reference a floor of a dungeon/cave that doesn't exist.