Steps to reproduce:

• create new world
• world type modern beta
• customize
• biome settings
• change Swampland -> Land to "Swamp" or Shrubland -> Land to "Desert"
• press Done twice
  It shows "saving world" then crashes.
  Any other biome changes work fine, even changing Swampland or Shrubland to something else works.

crash_swamp.txt
crash_desert.txt

Fixed with 3.0.4 and 4.0.0. Thanks!