pasted crash report

Hey... any idea what might be going on here?

Also using Lost Cities by McJty, which plays a large part in world gen.

Thanks, Shad. I dont know what biome since the world never loaded, but could also be Shinoow's Abyssalcraft biomes perhaps.

Huh. Neat. This might actually be a forge issue, either that or it's Lost Cities' issue.
This crash would also appear say if you used bonemeal on grass in that biome that you're in.
@McJty can you call Biome#addDefaultFlowers on your biome? Theres no way for me to check if that list is empty and when it's empty BlockGrass#grow throws this IllegalArgumentException.
Really, forge should be checking this, as this goes right through a forge method.

I'm gonna go see about a forge pr on this one.