Forge v14.23.4.2745-1.12.2
Actually Additions r138
I get this crash when generating a new world. Here's the crash report. Seems to have come back again.

So this isn't actually an issue I can reproduce, and the only way that crash report can happen is if another mod loads the variant enum before blocks are loaded, which is pretty bad... I've added a check into the enum that should crash properly allowing me to figure out what mod is causing this issue.