forge version: 35.1.15
mc: 1.16.4
crash-2020-12-28_23.08.49-fml.txt

👋 I just got the same crash. I'm guessing there's a race condition between this mod's "one time event receiver"'s unregistering itself and another mod doing the same or similar. Upon relaunching the game it loaded fine, though.

MC 1.16.4, Forge 35.1.28, Create 1.16.3-v0.3b

My stack trace: https://hastebin.com/qowufugace.txt

This was addressed in 0.3e, feel free to re-open if its still an issue. Cheers~