Up until now, my colony was able to have 6 kids naturally. A new pair of pawns I captured later also got together to have kids, but I keep getting this error from her. It has been retried multiple times, she's in fantastic health, never seen combat, etc. Any ideas?

Assigning active child backstory to adult pawn colonist.
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Warning (string)
RimWorld.Pawn_StoryTracker:set_Childhood (RimWorld.BackstoryDef)
(wrapper dynamic-method) RimWorld.PawnBioAndNameGenerator:RimWorld.PawnBioAndNameGenerator.FillBackstorySlotShuffled_Patch1 (Verse.Pawn,RimWorld.BackstorySlot,System.Collections.Generic.List`1<RimWorld.BackstoryCategoryFilter>,RimWorld.FactionDef,System.Nullable`1<RimWorld.BackstorySlot>)
RimWorld.LifeStageWorker_HumanlikeChild:Notify_LifeStageStarted (Verse.Pawn,RimWorld.LifeStageDef)
(wrapper dynamic-method) Verse.Pawn_AgeTracker:Verse.Pawn_AgeTracker.RecalculateLifeStageIndex_Patch2 (Verse.Pawn_AgeTracker)
Verse.Pawn_AgeTracker:set_AgeBiologicalTicks (long)
Verse.PawnGenerator:GenerateRandomAge (Verse.Pawn,Verse.PawnGenerationRequest)
Verse.PawnGenerator:TryGenerateNewPawnInternal (Verse.PawnGenerationRequest&,string&,bool,bool)
(wrapper dynamic-method) Verse.PawnGenerator:Verse.PawnGenerator.GenerateNewPawnInternal_Patch1 (Verse.PawnGenerationRequest&)
Verse.PawnGenerator:GenerateOrRedressPawnInternal (Verse.PawnGenerationRequest)
(wrapper dynamic-method) Verse.PawnGenerator:Verse.PawnGenerator.GeneratePawn_Patch3 (Verse.PawnGenerationRequest)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:BirthByPawnGenerationRequests (Verse.Pawn,Verse.Pawn,Verse.Pawn,System.Collections.Generic.List`1<Verse.PawnGenerationRequest>)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:Pawn_DoBirthSpawn_Pre (Verse.Pawn,Verse.Pawn)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.DoBirthSpawn_Patch2 (Verse.Pawn,Verse.Pawn)
Children.ChildrenHarmony/Hediff_Pregnant_PatchMCPrevent:Hediff_Pregnant_TickPre (Verse.Hediff_Pregnant)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.Tick_Patch1 (Verse.Hediff_Pregnant)
Verse.Pawn_HealthTracker:HealthTick ()
(wrapper dynamic-method) Verse.Pawn:Verse.Pawn.Tick_Patch2 (Verse.Pawn)
Verse.TickList:Tick ()
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.DoSingleTick_Patch2 (Verse.TickManager)
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.TickManagerUpdate_Patch2 (Verse.TickManager)
Verse.Game:UpdatePlay ()
(wrapper dynamic-method) Verse.Root_Play:Verse.Root_Play.Update_Patch1 (Verse.Root_Play)

Error while generating pawn. Rethrowing. Exception: System.NullReferenceException: Object reference not set to an instance of an object
at Verse.DefMap`2[D,V].get_Item (D def) [0x00000] in <95de19971c5d40878d8742747904cdcd>:0
at RimWorld.HistoryEventsManager.RecordEvent (RimWorld.HistoryEvent historyEvent, System.Boolean canApplySelfTookThoughts) [0x00056] in <95de19971c5d40878d8742747904cdcd>:0
at (wrapper dynamic-method) RimWorld.Pawn_IdeoTracker.RimWorld.Pawn_IdeoTracker.SetIdeo_Patch1(RimWorld.Pawn_IdeoTracker,RimWorld.Ideo)
at Verse.PawnGenerator.TryGenerateNewPawnInternal (Verse.PawnGenerationRequest& request, System.String& error, System.Boolean ignoreScenarioRequirements, System.Boolean ignoreValidator) [0x00632] in <95de19971c5d40878d8742747904cdcd>:0
at (wrapper dynamic-method) Verse.PawnGenerator.Verse.PawnGenerator.GenerateNewPawnInternal_Patch1(Verse.PawnGenerationRequest&)
at Verse.PawnGenerator.GenerateOrRedressPawnInternal (Verse.PawnGenerationRequest request) [0x00188] in <95de19971c5d40878d8742747904cdcd>:0
at (wrapper dynamic-method) Verse.PawnGenerator.Verse.PawnGenerator.GeneratePawn_Patch3(Verse.PawnGenerationRequest)
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
(wrapper dynamic-method) Verse.PawnGenerator:Verse.PawnGenerator.GeneratePawn_Patch3 (Verse.PawnGenerationRequest)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:BirthByPawnGenerationRequests (Verse.Pawn,Verse.Pawn,Verse.Pawn,System.Collections.Generic.List`1<Verse.PawnGenerationRequest>)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:Pawn_DoBirthSpawn_Pre (Verse.Pawn,Verse.Pawn)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.DoBirthSpawn_Patch2 (Verse.Pawn,Verse.Pawn)
Children.ChildrenHarmony/Hediff_Pregnant_PatchMCPrevent:Hediff_Pregnant_TickPre (Verse.Hediff_Pregnant)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.Tick_Patch1 (Verse.Hediff_Pregnant)
Verse.Pawn_HealthTracker:HealthTick ()
(wrapper dynamic-method) Verse.Pawn:Verse.Pawn.Tick_Patch2 (Verse.Pawn)
Verse.TickList:Tick ()
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.DoSingleTick_Patch2 (Verse.TickManager)
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.TickManagerUpdate_Patch2 (Verse.TickManager)
Verse.Game:UpdatePlay ()
(wrapper dynamic-method) Verse.Root_Play:Verse.Root_Play.Update_Patch1 (Verse.Root_Play)

BirthByPawnGenerationRequests: error: Object reference not set to an instance of an object
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
ChdLogger.ChdLogging:LogError (string)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:BirthByPawnGenerationRequests (Verse.Pawn,Verse.Pawn,Verse.Pawn,System.Collections.Generic.List`1<Verse.PawnGenerationRequest>)
Children.ChildrenHarmony/Hediff_Pregnant_Patch2:Pawn_DoBirthSpawn_Pre (Verse.Pawn,Verse.Pawn)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.DoBirthSpawn_Patch2 (Verse.Pawn,Verse.Pawn)
Children.ChildrenHarmony/Hediff_Pregnant_PatchMCPrevent:Hediff_Pregnant_TickPre (Verse.Hediff_Pregnant)
(wrapper dynamic-method) Verse.Hediff_Pregnant:Verse.Hediff_Pregnant.Tick_Patch1 (Verse.Hediff_Pregnant)
Verse.Pawn_HealthTracker:HealthTick ()
(wrapper dynamic-method) Verse.Pawn:Verse.Pawn.Tick_Patch2 (Verse.Pawn)
Verse.TickList:Tick ()
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.DoSingleTick_Patch2 (Verse.TickManager)
(wrapper dynamic-method) Verse.TickManager:Verse.TickManager.TickManagerUpdate_Patch2 (Verse.TickManager)
Verse.Game:UpdatePlay ()
(wrapper dynamic-method) Verse.Root_Play:Verse.Root_Play.Update_Patch1 (Verse.Root_Play)