So i noticed that the polar exterminators den would often spawn outside of the snow biome when Advanced world gen is enabled. This is the first time it actually failed on worldgen though.

Logs:
[22:04:08.998] [WorldGen/WARN] [tML]: Silently Caught Exception:
System.IndexOutOfRangeException: Index was outside the bounds of the array.
at QwertyMod.Common.FrozenDen.GenerateDen(Int32 x, Int32 y) in QwertyMod/Common/FrozenDen.cs:line 66
at QwertyMod.Common.FrozenDen.RunDenGenerator() in QwertyMod/Common/FrozenDen.cs:line 319
at DMD<System.Void Terraria.WorldBuilding.WorldGenerator:GenerateWorld(Terraria.WorldBuilding.GenerationProgress)>(WorldGenerator this, GenerationProgress progress)
at AdvancedWorldGen.BetterVanillaWorldGen.Replacer.ChangeWeights(orig_GenerateWorld orig, WorldGenerator self, GenerationProgress progress) in AdvancedWorldGen\BetterVanillaWorldGen\Replacer.Hooks.cs:line 136
at Hook<System.Void AdvancedWorldGen.BetterVanillaWorldGen.Replacer::ChangeWeights(Terraria.WorldBuilding.On_WorldGenerator+orig_GenerateWorld,Terraria.WorldBuilding.WorldGenerator,Terraria.WorldBuilding.GenerationProgress)>(WorldGenerator , GenerationProgress )
at SyncProxy<System.Void Terraria.WorldBuilding.WorldGenerator:GenerateWorld(Terraria.WorldBuilding.GenerationProgress)>(WorldGenerator , GenerationProgress )
at Terraria.WorldGen.GenerateWorld_RunTasksAndFinish(Int32 seed, Stopwatch generationStopwatch, GenerationProgress customProgressObject) in tModLoader\Terraria\WorldGen.cs:line 14025
at DMD<System.Void Terraria.WorldGen:GenerateWorld(System.Int32, Terraria.WorldBuilding.GenerationProgress)>(Int32 seed, GenerationProgress customProgressObject)
at SyncProxy<System.Void Terraria.WorldGen:GenerateWorld(System.Int32, Terraria.WorldBuilding.GenerationProgress)>(Int32 , GenerationProgress )
at DMD<System.Void Terraria.WorldGen:do_worldGenCallBack(System.Object)>(Object threadContext)
at AdvancedWorldGen.UI.UIChanger.<>c__DisplayClass10_0.<ThreadifyWorldGen>b__0() in AdvancedWorldGen\UI\UIChanger.cs:line 38
at System.Threading.Thread.StartHelper.Callback(Object state)
at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)

[22:04:08.999] [WorldGen/ERROR] [tML]: A problem was encountered during world generation
Carving out a beast's den!
System.IndexOutOfRangeException: Index was outside the bounds of the array.
at QwertyMod.Common.FrozenDen.GenerateDen(Int32 x, Int32 y) in QwertyMod/Common/FrozenDen.cs:line 66
at QwertyMod.Common.FrozenDen.RunDenGenerator() in QwertyMod/Common/FrozenDen.cs:line 319
at DMD<System.Void Terraria.WorldBuilding.WorldGenerator:GenerateWorld(Terraria.WorldBuilding.GenerationProgress)>(WorldGenerator this, GenerationProgress progress)
[22:04:19.017] [Main Thread/DEBUG] [tML]: Process.Start (UseShellExecute = True): "tModLoader-Logs"