Looks like any mod that uses tutorial from Animal Variety Coats mod will cause the same error with Faction Blender mod:

Exception filling window for Verse.Dialog_DebugOptionListLister: System.NullReferenceException: Object reference not set to an instance of an object
at RimWorld.IncidentWorker_Raid.TryExecuteWorker (RimWorld.IncidentParms parms) [0x00000] in <filename unknown>:0
at RimWorld.IncidentWorker_RaidEnemy.TryExecuteWorker (RimWorld.IncidentParms parms) [0x00000] in <filename unknown>:0
at RimWorld.IncidentWorker.TryExecute (RimWorld.IncidentParms parms) [0x00000] in <filename unknown>:0
at Verse.Dialog_DebugActionsMenu.DoRaid (RimWorld.IncidentParms parms) [0x00000] in <filename unknown>:0
at Verse.Dialog_DebugActionsMenu+<DoListingItems_MapActions>c__AnonStorey8+<DoListingItems_MapActions>c__AnonStoreyA+<DoListingItems_MapActions>c__AnonStoreyB+<DoListingItems_MapActions>c__AnonStoreyC.<>m__0 () [0x00000] in <filename unknown>:0
at Verse.Dialog_DebugOptionLister.DebugAction (System.String label, System.Action action) [0x00000] in <filename unknown>:0
at Verse.Dialog_DebugOptionListLister.DoListingItems () [0x00000] in <filename unknown>:0
at Verse.Dialog_OptionLister.DoWindowContents (Rect inRect) [0x00000] in <filename unknown>:0
at Verse.Window+<WindowOnGUI>c__AnonStorey0.<>m__0 (Int32 x) [0x00000] in <filename unknown>:0
Verse.Log:Error(String, Boolean)
Verse.<WindowOnGUI>c__AnonStorey0:<>m__0(Int32)
UnityEngine.GUI:CallWindowDelegate(WindowFunction, Int32, Int32, GUISkin, Int32, Single, Single, GUIStyle)

Reproduce:
Spawn 4-8 raids in a row (4000-5000 points, Immediate attack, edgewalking) from both new mixed faction blender factions. One of the raids will cause that error, probably when animal variations dll kicks in.