If you use incorrect biome ID in a superflat preset, the world generation will stuck in "Converting world". It's difficult to close Minecraft without the Task manager.

How to reproduce :
1. Create new world
2. Go to "More world options"
3. Select superflat world type
4. Click on "Customize"
5. In the preset box, type 2;7;x;village
Replace x by any invalid biome id (below 0 or above 22)
6. Click on "Use preset"
7. Click on "Done"
8. Create the new world

EDIT 1:I get a crash report