If it is saying there's a MAG in your gun but you know there's not try putting the gun on the floor and drag a new clip to the slot on the gun or equipped it and drag a new clip to the slot on the gun. Also once you to do that see if you can then now hit Y to reload or swap mags. Not a fix but if you mess with it you can make it work again. But please fix this bug it always happens at the worst time.