Hi what does this mean?

System.NullReferenceException: A null value was found where an object instance was required.
#0: 0x00051 stfld.o in Sims3.Gameplay.Actors.Sim+GhostReaction:Run () ()
#1: 0x000d7 callvirt in Sims3.Gameplay.Interactions.Sims3.Gameplay.Interactions.InteractionInstance:RunInteractionWithoutCleanup () ()
#2: 0x00344 callvirt in Sims3.Gameplay.ActorSystems.Sims3.Gameplay.ActorSystems.InteractionQueue:ProcessOneInteraction () ()
#3: 0x00118 callvirt in Sims3.Gameplay.Actors.Sims3.Gameplay.Actors.Sim:DoInteraction () ()
#4: 0x00013 call in Sims3.Gameplay.Actors.Sims3.Gameplay.Actors.Sim:Simulate () ()
#5: 0x00027 leave.s in ScriptCore.ScriptCore.ScriptProxy:Simulate () ()

I keep getting it at night on full moons when there are ghosts haunting my house but only with one of my sims. Errortrap randomly resets the sim and logs this error. There are multiple sims in the house but this only happens with her