The bug

When using a non-[a-z0-9/._-]-character as biome in a superflat preset, the game crashes.

How to reproduce

1. Open MC 1.14.4 or later
2. Create new superflat world
3. Choose a preset that uses a non-[a-z0-9/._-] character as biome (e.g. minecraft:air;*;)
4. Click Done (or whatever button it was)
   → The game crashes instead of giving a proper error message