That’s extremely strange. I managed to get a screencap of the error if it helps any.
I’ll dig around in the lua code and see if I can come up with anything.
One thing I did notice is that the hearthling a) loses clothes and the arcanist outfit appears close but not on them and b) They never finish promoting. The errors also seem to never stop, so that’s not a good sign, haha.
Edit: it seems @Froggy’s Stonehearth Cafe mod had the same problem a few times. Guess I’ll dig around in there as well. Fun times are ahead! I think I’ve learned more about coding in the past three days than I have every time I’ve gone on Codecademy. 
Edit the Second: So, nothing I’ve done within the past few hours has shed any light on this situation, the closest I got was a Github link on the Stonehearth Cafe mod page linked above, which had a couple of lines changed in _description.json which were already changed in arcanist_description.json. So, back to the drawing board. It’s thundering here, so I might get an actual drawing board in case the power goes out. Woo!
Edit the Third: I’m offically out of ideas. If anyone is willing to take a look at some butchered code for me, I’d be grateful. What I have is here. Go nuts I guess, haha.