Forge 2796
Mod 0.9.3
Minecraft 1.12.2

Server crashes with this crash log https://hastebin.com/uqoxesubet.rb

Just curious.. Do you have worldgen disabled?

Also, worth noting the crash doesn't happen with the previous version.

I do. I want to give people the option of planting dynamic trees, but not having them generate in the world.

I know exactly what happened. I've already fixed it in the dev version. For now you can downgrade to the previous version since you won't really have any changes considering that you're running without world gen anyway.