browser_side: print a warning when trying to access a non existing contact box: This occurs when you receive updatePresence of a MUC occupant before Libervia knows that this is a MUC. Actually you shouldn't receive presence update for the occupant before the MUC widget is displayed, this means a loss of information. One way to fix it is to modify (un)availableReceived in sat.core.xmpp: check for the entity data "type" or make a disco request if it doesn't exist, and an attribute to presenceUpdate to tell the frontend if we have a MUC occupant or a roster contact.