messageTray: Only try to focus the summary when summoning the tray by <Super>M

Otherwise, when closing the tray, we'll try to focus an actor, which will
focus the stage window, which will drop the focus from whatever window we
already had focused.

https://bugzilla.gnome.org/show_bug.cgi?id=710347

js/ui/messageTray.js

@@ -2212,7 +2212,10 @@ const MessageTray = new Lang.Class({
     },
 
     toggleAndNavigate: function() {
-        if (this.toggle())
+        if (!this.toggle())
+            return;
+
+        if (this._traySummoned)
             this._summary.navigate_focus(null, Gtk.DirectionType.TAB_FORWARD, false);
     },