Steps to reproduce the crash:

1. Compile the latest master (as of f3da617)
2. Hold biplane from ye olde pack in the player hand

• Forge: v14.23.5.2854
• Flan's Mod: f3da617

Crash Log
gist

May be related to #1193

I suspect you've compiled the mod with Java 9+ and then it's failing when you try to run it on Java 8

Can you confirm this?

You're correct. Java path was set to version 11. Compiling with Java 8 fixes the problem.

I should have checked, thank you for your time.

Closing.