has an error,can you help me?

System.NullReferenceException: Object reference not set to an instance of an object at rjw.PATCH_Hediff_Pregnant_DoBirthSpawn.on_begin_DoBirthSpawn (Verse.Pawn&,Verse.Pawn&) <0x00451> at (wrapper dynamic-method) Verse.Hediff_Pregnant.DoBirthSpawn_Patch2 (Verse.Pawn,Verse.Pawn) <0x0004e> at Verse.Hediff_Pregnant.Tick () <0x0042e> at Verse.Pawn_HealthTracker.HealthTick () <0x00077> at Verse.Pawn.Tick () <0x001a9> at RimWorld.Planet.WorldPawns.WorldPawnsTick () <0x00088> at RimWorld.Planet.World.WorldTick () <0x00017> at Verse.TickManager.DoSingleTick () <0x0024c>