Title is self explanatory. Every time after i uninstalled the mod i crash, but reinstalling it fixes it. i just want a superflat/biomes o'plenty world.

Hey @AyumuTheDreamer, thanks for reporting this. Haven't tested this for a while, may be because of custom biomes being used that don't replace to a vanilla biome, which would mean the biomes would be absent from the registry and the world won't work. Please ask on the OTG Discord as well, people there may be able to help you more quickly.

Title is self explanatory. Every time after i uninstalled the mod i crash, but reinstalling it fixes it. i just want a superflat/biomes o'plenty world.

You mean OTG mod, right?
If yes, then #197

Ah ok, so removing OTG-Core fixed it?

Oh! I fixed it by removing the folder called 1.12.2.

Ok, glad it's fixed, thanks for reporting :).