Ok, good to know that the inventory system, etc can be cleanly yanked out for right now!
As for running into this, I was just learning the atlas/font maker tools and trying to get up to speed on the proper work flow. I was blindly doing a bunch of odd actions like trying to duplicate an atlas and its resources to make a quick backup of it, then manually fixing up the references to the duplicate resources, trying to merge an existing atlas and sprites into a new larger atlas with existing sprites, pulling in generated font data, etc. There was probably a few automatic unity code reloads from external code modifications while the game was running, too. You're right, that's probably what triggered the exceptions since that always seems to cause issues in any project. I'll definitely post repro steps if I can nail it down to an exact process.
Thanks!