Problem as per title. ambush symbol appears and another empty caravan is generated but real caravan moves on without interruption. I beleive this is the relevant error message:
Exception from long event: System.NullReferenceException: Object reference not set to an instance of an object
at RimWorld.Planet.CaravansBattlefield.PostMapGenerate () [0x0000c] in <d72310b4d8f64d25aee502792b58549f>:0
at (wrapper dynamic-method) Verse.MapGenerator.Verse.MapGenerator.GenerateMap_Patch0(Verse.IntVec3,RimWorld.Planet.MapParent,Verse.MapGeneratorDef,System.Collections.Generic.IEnumerable`1<Verse.GenStepWithParams>,System.Action`1<Verse.Map>)
at Verse.GetOrGenerateMapUtility.GetOrGenerateMap (System.Int32 tile, Verse.IntVec3 size, RimWorld.WorldObjectDef suggestedMapParentDef) [0x0006c] in <d72310b4d8f64d25aee502792b58549f>:0
at RimWorld.Planet.CaravanIncidentUtility.GetOrGenerateMapForIncident (RimWorld.Planet.Caravan caravan, Verse.IntVec3 size, RimWorld.WorldObjectDef suggestedMapParentDef) [0x00012] in <d72310b4d8f64d25aee502792b58549f>:0
at RimWorld.Planet.CaravanIncidentUtility.SetupCaravanAttackMap (RimWorld.Planet.Caravan caravan, System.Collections.Generic.List`1[T] enemies, System.Boolean sendLetterIfRelatedPawns) [0x0001d] in <d72310b4d8f64d25aee502792b58549f>:0
at (wrapper dynamic-method) RimWorld.IncidentWorker_Ambush.RimWorld.IncidentWorker_Ambush.DoExecute_Patch0(RimWorld.IncidentWorker_Ambush,RimWorld.IncidentParms,System.Collections.Generic.List`1<Verse.Pawn>,Verse.IntVec3)
at RimWorld.IncidentWorker_Ambush+<>c__DisplayClass4_0.<TryExecuteWorker>b__0 () [0x00000] in <d72310b4d8f64d25aee502792b58549f>:0
at Verse.LongEventHandler.UpdateCurrentSynchronousEvent (System.Boolean& sceneChanged) [0x0001d] in <d72310b4d8f64d25aee502792b58549f>:0
Verse.Log:Error(String, Boolean)
Verse.LongEventHandler:UpdateCurrentSynchronousEvent(Boolean&)
Verse.LongEventHandler:LongEventsUpdate(Boolean&)
Verse.Root:Verse.Root.Update_Patch1(Root)
Verse.Root_Play:Update()
Exception from long event: System.NullReferenceException: Object reference not set to an instance of an object
at RimWorld.Planet.CaravansBattlefield.PostMapGenerate () [0x0000c] in <d72310b4d8f64d25aee502792b58549f>:0
at (wrapper dynamic-method) Verse.MapGenerator.Verse.MapGenerator.GenerateMap_Patch0(Verse.IntVec3,RimWorld.Planet.MapParent,Verse.MapGeneratorDef,System.Collections.Generic.IEnumerable`1<Verse.GenStepWithParams>,System.Action`1<Verse.Map>)
at Verse.GetOrGenerateMapUtility.GetOrGenerateMap (System.Int32 tile, Verse.IntVec3 size, RimWorld.WorldObjectDef suggestedMapParentDef) [0x0006c] in <d72310b4d8f64d25aee502792b58549f>:0
at RimWorld.Planet.CaravanIncidentUtility.GetOrGenerateMapForIncident (RimWorld.Planet.Caravan caravan, Verse.IntVec3 size, RimWorld.WorldObjectDef suggestedMapParentDef) [0x00012] in <d72310b4d8f64d25aee502792b58549f>:0
at RimWorld.Planet.CaravanIncidentUtility.SetupCaravanAttackMap (RimWorld.Planet.Caravan caravan, System.Collections.Generic.List`1[T] enemies, System.Boolean sendLetterIfRelatedPawns) [0x0001d] in <d72310b4d8f64d25aee502792b58549f>:0
at (wrapper dynamic-method) RimWorld.IncidentWorker_Ambush.RimWorld.IncidentWorker_Ambush.DoExecute_Patch0(RimWorld.IncidentWorker_Ambush,RimWorld.IncidentParms,System.Collections.Generic.List`1<Verse.Pawn>,Verse.IntVec3)
at RimWorld.IncidentWorker_Ambush+<>c__DisplayClass4_0.<TryExecuteWorker>b__0 () [0x00000] in <d72310b4d8f64d25aee502792b58549f>:0
at Verse.LongEventHandler.UpdateCurrentSynchronousEvent (System.Boolean& sceneChanged) [0x0001d] in <d72310b4d8f64d25aee502792b58549f>:0
Verse.Log:Error(String, Boolean)
Verse.LongEventHandler:UpdateCurrentSynchronousEvent(Boolean&)
Verse.LongEventHandler:LongEventsUpdate(Boolean&)
Verse.Root:Verse.Root.Update_Patch1(Root)
Verse.Root_Play:Update()