So, uh. I'll preface this by saying I understand this is all experimental stuff. This is partly why I chose to open an issue about it. Also, I've removed OptiForge (and the two dependencies - Mixins and OptiFine) and it ran perfectly without issue. Placing them back in, reproduced a duplicate crash report.

This is the crash report: https://pastebin.com/ijqAwnhF

How it crashes, is I load up a world and then press F3 to open the debug menu. Simple as that, it crashes to desktop with the above crash report.

The report is blaming Xaero's minimap for the crash, so I've bugged him about this as well. However, since it does only occur when I add OptiForge, I figured I should give you a poke as well, Zeker.

Additional information:

• Forge version 31.1.34.
• OptiFine version OptiFine_1.15.2_HD_U_G1_pre12.
• OptiForge version 1.15.2-0.1.7
• Mixins version MixinLoader-1.1-SNAPSHOT.
• Java version 1.8.0_241.
• Other mods:
  • U Team Core u_team_core-1.15.2-2.11.0.156.
  • Server Tab Info ServerTabInfo-1.15.2-1.2.6.
  • Xaero's Minimap Xaeros_Minimap_20.6.0_Forge_1.15.2.
  • AppleSkin AppleSkin-mc1.15.2-forge-1.0.13.
  • Xaero's World Map XaerosWorldMap_1.5.6_Forge_1.15.2.
  • Just Enough Items jei-1.15.2-6.0.0.2.

Looks like it was indeed an issue on Xaero's end, so I'll go ahead and close this. Xaero confirmed it on his forum thread.

As a side note (and an afterthought), I seem to have misread the crash report and said it was the minimap. It's actually the world map.