Crash report: https://pastebin.com/gCgEYX9t

Setting runewood rarity any lower than 0.005 causes the above crash. 0.004 and 0.0005 are examples of what I tried.

Setting it back to 0.005 resolved the behavior.

This crash occurs when you click the single player button of the main menu. How tree rarity causes this when worlds haven't even loaded yet is beyond me

The expectation is that the user could use a lower rarity than 0.005 as this still seems to be roughly 1 in 80 chunks (eyeballing)

Forge: 39.0.79