SlayTheSpire.log
I believe that this crash is an issue originating from here (not 100% certain though)
https://github.com/Tempus/SlayTheStreamer/blob/master/src/main/java/chronometry/patches/BossChoicePatch.java#L109
It's also possible that the boss selection is also bugged but I'm not certain as I don't have a way to get to the boss room without the game selecting a boss to fight against. (I tried swapping between the unmodded and modded versions of the same save... it didn't work as the boss was selected as soon as I hit the unmodded game)