I tried to create a flat singleplayer world as my client crashed.

How to reproduce:

• Singleplayer
• Create New World
• More World Options
• World Type -> Superflat
• Customize
• Presets
• (The Void)
• Type into the preset field: minecraft:yellow_terracotta; (the block does not matter)
• Click on use Preset
• Done -> The game crashes

This crash is caused by the missing biome in the preset string! The game should just default the biome to plains and not throw an IllegalStateException (see crash.txt).