mpris: Prevent DBus race when creating media players

Sometimes an MPRIS media player will create and then destroy an object
before the signals that handle the object's destruction can be created.
This verifies that the object still exists after the necessary signals
have been created.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2776

js/ui/mpris.js

@@ -173,6 +173,11 @@ var MprisPlayer = class MprisPlayer {
                 if (!this._mprisProxy.g_name_owner)
                     this._close();
             });
+        // It is possible for the bus to disappear before the previous signal
+        // is connected, so we must ensure that the bus still exists at this
+        // point.
+        if (!this._mprisProxy.g_name_owner)
+            this._close();
     }
 
     _onPlayerProxyReady() {