Use async function to load DBUS xml file

extension.js

@@ -555,31 +555,19 @@ class MonitorWidget extends PanelMenu.Button {
     }
 });
 
-// From https://github.com/ubuntu/gnome-shell-extension-appindicator/blob/master/interfaces.js
-// loads a xml file into an in-memory string
-function loadInterfaceXml(extension, filename) {
+async function loadInterfaceXml(extension, filename) {
     const interfacesDir = extension.dir.get_child('.');
 
     const file = interfacesDir.get_child(filename);
+    const fd = Gio.File.new_for_path(file.get_path());
+    const [contents, etag] = await fd.load_contents_async(null);
 
-    let [result, contents] = GLib.file_get_contents(file.get_path());
+    const decoder = new TextDecoder('utf-8');
+    const decoded_content = decoder.decode(contents);
 
-    if (result) {
-        // HACK: The "" + trick is important as hell because file_get_contents returns
-        // an object (WTF?) but Gio.makeProxyWrapper requires `typeof() == "string"`
-        // Otherwise, it will try to check `instanceof XML` and fail miserably because there
-        // is no `XML` on very recent SpiderMonkey releases (or, if SpiderMonkey is old enough,
-        // will spit out a TypeError soon).
-        if (contents instanceof Uint8Array)
-        {
-            const decoder = new TextDecoder();
-            contents = decoder.decode(contents);
-        }
-        const res = `<node>${contents}</node>`;
-        return res;
-    } else {
-        throw new Error(`Generic monitor: Could not load file: ${filename}`);
-    }
+    const res = `<node>${decoded_content}</node>`;
+    
+    return res;
 }
 
 class GenericMonitorDBUS {
@@ -588,9 +576,15 @@ class GenericMonitorDBUS {
         this.monitor_groups = {};
         this.actor_clicked = {};
         this.ClutterSettings = Clutter.Settings.get_default();
-        this._dbusImpl = Gio.DBusExportedObject.wrapJSObject(loadInterfaceXml(extension, 'dbus.xml'), this);
-        this._dbusImpl.export(Gio.DBus.session, '/com/soutade/GenericMonitor');
-        this._dbusImpl.emit_signal('onActivate', null);
+        this._dbusImpl =  null;
+        loadInterfaceXml(extension, 'dbus.xml')
+            .then( (xml) =>
+                {
+                    this._dbusImpl = Gio.DBusExportedObject.wrapJSObject(xml, this);
+                    this._dbusImpl.export(Gio.DBus.session, '/com/soutade/GenericMonitor');
+                    this._dbusImpl.emit_signal('onActivate', null);
+                })
+            .catch((e) => { this.logger.error(e); });
     }
 
     emitSignal(name, value) {