Thanks for the repro steps. Unfortunately I don't think there is anything that I can do. PhotonView 124 is the dropped assault rifle and the error that we are receiving is called before the character controller has a chance to react. The next frame the add-on receives the instantiation event and assigns the ViewID to 124. PUN says the following in the notes about the log so I think that we can ignore it since everything is functioning correctly:
Show only errors. Minimal output. Note: Some might be "runtime errors" which you have to expect.