Exception while generating thing set: System.NullReferenceException: Object reference not set to an instance of an object
  at RimWorld.StockGeneratorUtility+<TryMakeForStock>d__0.MoveNext () [0x00022] in <00638943358e4807bd44cd7bc8f572da>:0 
  at RimWorld.StockGenerator_SingleDef+<GenerateThings>d__1.MoveNext () [0x0007b] in <00638943358e4807bd44cd7bc8f572da>:0 
  at RimWorld.ThingSetMaker_TraderStock.Generate (RimWorld.ThingSetMakerParams parms, System.Collections.Generic.List`1[T] outThings) [0x000e4] in <00638943358e4807bd44cd7bc8f572da>:0 
  at RimWorld.ThingSetMaker.Generate (RimWorld.ThingSetMakerParams parms) [0x00019] in <00638943358e4807bd44cd7bc8f572da>:0 
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
RimWorld.ThingSetMaker:Generate (RimWorld.ThingSetMakerParams)
RimWorld.TradeShip:GenerateThings ()
CallTradeShips.JobDriver_CallTradeShip:<MakeNewToils>b__1_0 ()
Verse.AI.JobDriver:TryActuallyStartNextToil ()
(wrapper dynamic-method) Verse.AI.JobDriver:Verse.AI.JobDriver.ReadyForNextToil_Patch1 (Verse.AI.JobDriver)
Verse.AI.JobDriver:Notify_PatherArrived ()
Verse.AI.Pawn_PathFollower:PatherArrived ()
Verse.AI.Pawn_PathFollower:StartPath (Verse.LocalTargetInfo,Verse.AI.PathEndMode)
Verse.AI.Toils_Goto/<>c__DisplayClass1_0:<GotoThing>b__0 ()
Verse.AI.JobDriver:TryActuallyStartNextToil ()
(wrapper dynamic-method) Verse.AI.JobDriver:Verse.AI.JobDriver.ReadyForNextToil_Patch1 (Verse.AI.JobDriver)
(wrapper dynamic-method) Verse.AI.Pawn_JobTracker:Verse.AI.Pawn_JobTracker.StartJob_Patch1 (Verse.AI.Pawn_JobTracker,Verse.AI.Job,Verse.AI.JobCondition,Verse.AI.ThinkNode,bool,bool,Verse.ThinkTreeDef,System.Nullable`1<Verse.AI.JobTag>,bool,bool)
Verse.AI.Pawn_JobTracker:TryFindAndStartJob ()
(wrapper dynamic-method) Verse.AI.Pawn_JobTracker:Verse.AI.Pawn_JobTracker.EndCurrentJob_Patch1 (Verse.AI.Pawn_JobTracker,Verse.AI.JobCondition,bool,bool)
Verse.AI.Pawn_JobTracker:JobTrackerTick ()
Verse.Pawn:Tick ()
(wrapper dynamic-method) Verse.TickList:Verse.TickList.Tick_Patch2 (Verse.TickList)
Verse.TickManager:DoSingleTick ()
Verse.TickManager:TickManagerUpdate ()
Verse.Game:UpdatePlay ()
Verse.Root_Play:Update ()