Error when you try to regenerate world

#LUA ERROR stack traceback:
@scripts/map/level.lua:204 in (method) ChooseSetPieces (Lua) <160-215>
   self =
      chosen_tasks = table: 06B37470
      max_playlist_position = 999
      tasks = table: 06B375D8
      set_pieces = table: 06B371A0
      numoptionaltasks = 5
      numrandom_set_pieces = 4
      ocean_population = table: 06B37560
      substitutes = table: 220A21C0
      required_setpieces = table: 220A1BA8
      ordered_story_setpieces = table: 06B3A5F8
      min_playlist_position = 0
      override_level_string = false
      id = SURVIVAL_TOGETHER
      required_prefabs = table: 06B37240
      desc = The standard Don't Starve experience.
      hideminimap = false
      valid_start_tasks = table: 06B381E0
      optionaltasks = table: 06B37510
      name = Together
      version = 4
      location = forest
      ocean_prefill_setpieces = table: 06B37268
      random_set_pieces = table: 220A1838
      overrides = table: 220A20F8
   tasks = table: 07F6EB28
   name = Level2TwigsBoon
   choicedata = table: 07F6E8F8
   found = false
   idx = table: 07F6FCF8
   availabletasks = table: 07F70298
   choices = table: 07F702C0
   count = 1
   idx_choice = 17
   choice = nil
@scripts/worldgen_main.lua:380 in () ? (Lua) <355-475>
   debug = false
   world_gen_data = table: 220A0EB0
   level = table: 06B36778
   prefab = forest
@=(tail call):-1 in ()  (tail) <-1--1>
@=(tail call):-1 in ()  (tail) <-1--1>
[00:03:56]: #[string "scripts/map/level.lua"]:204: attempt to index field '?' (a nil value)
#LUA ERROR stack traceback:
@scripts/map/level.lua:204 in (method) ChooseSetPieces (Lua) <160-215>
   self =
      chosen_tasks = table: 06B37470
      max_playlist_position = 999
      tasks = table: 06B375D8
      set_pieces = table: 06B371A0
      numoptionaltasks = 5
      numrandom_set_pieces = 4
      ocean_population = table: 06B37560
      substitutes = table: 220A21C0
      required_setpieces = table: 220A1BA8
      ordered_story_setpieces = table: 06B3A5F8
      min_playlist_position = 0
      override_level_string = false
      id = SURVIVAL_TOGETHER
      required_prefabs = table: 06B37240
      desc = The standard Don't Starve experience.
      hideminimap = false
      valid_start_tasks = table: 06B381E0
      optionaltasks = table: 06B37510
      name = Together
      version = 4
      location = forest
      ocean_prefill_setpieces = table: 06B37268
      random_set_pieces = table: 220A1838
      overrides = table: 220A20F8
   tasks = table: 07F6EB28
   name = Level2TwigsBoon
   choicedata = table: 07F6E8F8
   found = false
   idx = table: 07F6FCF8
   availabletasks = table: 07F70298
   choices = table: 07F702C0
   count = 1
   idx_choice = 17
   choice = nil
@scripts/worldgen_main.lua:380 in () ? (Lua) <355-475>
   debug = false
   world_gen_data = table: 220A0EB0
   level = table: 06B36778
   prefab = forest
@=(tail call):-1 in ()  (tail) <-1--1>
@=(tail call):-1 in ()  (tail) <-1--1>
[00:03:56]: Error loading worldgen_main.lua
[00:03:56]: WorldSim::SimThread::Main() ERROR
[00:03:56]: WorldSim::SimThread::Main() complete
[00:03:56]: Worldgen had an error, displaying...	
[00:03:56]: #[string "scripts/map/level.lua"]:204: attempt to index field '?' (a nil value)
#LUA ERROR stack traceback:
@scripts/map/level.lua:204 in (method) ChooseSetPieces (Lua) <160-215>
   self =
      chosen_tasks = table: 06B37470
      max_playlist_position = 999
      tasks = table: 06B375D8
      set_pieces = table: 06B371A0
      numoptionaltasks = 5
      numrandom_set_pieces = 4
      ocean_population = table: 06B37560
      substitutes = table: 220A21C0
      required_setpieces = table: 220A1BA8
      ordered_story_setpieces = table: 06B3A5F8
      min_playlist_position = 0
      override_level_string = false
      id = SURVIVAL_TOGETHER
      required_prefabs = table: 06B37240
      desc = The standard Don't Starve experience.
      hideminimap = false
      valid_start_tasks = table: 06B381E0
      optionaltasks = table: 06B37510
      name = Together
      version = 4
      location = forest
      ocean_prefill_setpieces = table: 06B37268
      random_set_pieces = table: 220A1838
      overrides = table: 220A20F8
   tasks = table: 07F6EB28
   name = Level2TwigsBoon
   choicedata = table: 07F6E8F8
   found = false
   idx = table: 07F6FCF8
   availabletasks = table: 07F70298
   choices = table: 07F702C0
   count = 1
   idx_choice = 17
   choice = nil
@scripts/worldgen_main.lua:380 in () ? (Lua) <355-475>
   debug = false
   world_gen_data = table: 220A0EB0
   level = table: 06B36778
   prefab = forest
@=(tail call):-1 in ()  (tail) <-1--1>
@=(tail call):-1 in ()  (tail) <-1--1>	
[00:03:56]: Warning: Widget:SetFocusFromChild is happening on a widget outside of the screen/widget hierachy. This will cause focus moves to fail. Is 	ScriptErrorWidget	not a screen?	
[00:03:56]: stack traceback:
	scripts/widgets/widget.lua:605 in (method) SetFocusFromChild (Lua) <602-627>
	scripts/widgets/widget.lua:624 in (method) SetFocusFromChild (Lua) <602-627>
	scripts/widgets/widget.lua:624 in (method) SetFocusFromChild (Lua) <602-627>
	scripts/widgets/widget.lua:656 in (method) SetFocus (Lua) <629-665>
	scripts/widgets/menu.lua:83 in (method) SetFocus (Lua) <74-85>
	scripts/widgets/scripterrorwidget.lua:109 in (method) OnUpdate (Lua) <102-119>
	scripts/update.lua:95 in () ? (Lua) <33-134>