shellDBus: Properly return from D-Bus methods

When implementing a D-Bus method synchronously, regular JS methods are
used. That is, whatever the method returns is passed as return value
to the method invocation.

However for asynchronous implementations, we need to explicitly return
a value to the invocation, otherwise the caller will wait until it times
out eventually.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1891>

js/ui/shellDBus.js

@@ -217,11 +217,13 @@ var GnomeShell = class {
         let sender = invocation.get_sender();
         let [dict] = params;
         Main.osdMonitorLabeler.show(sender, dict);
+        invocation.return_value(null);
     }
 
     HideMonitorLabelsAsync(params, invocation) {
         let sender = invocation.get_sender();
         Main.osdMonitorLabeler.hide(sender);
+        invocation.return_value(null);
     }
 
     _checkOverviewVisibleChanged() {