Null exception

[FG28_Kodiak]

For me it seems the error is in

 private void damageAiControlledPlane(AiActor actorMain)
    {        
        foreach (AiActor actor in actorMain.Group().GetItems())
        {
            if (actor == null || !(actor is AiAircraft))
                return;
            AiAircraft aircraft = (actor as AiAircraft);
            if (!isAiControlledPlane(aircraft))
                return;
            if (aircraft == null)
                return;
            (actor as AiAircraft).Destroy();             
        }
    }

Why you use the Group to destroy a single Airplane? Do you test the Group for null value? It make no sense to me to use the Group in this case.