Use generic get_object

Author: UrtsiSantsi

src/Screencast/main.ts

@@ -25,7 +25,7 @@ Gio._promisify(Xdp.Session.prototype, "start", "start_finish");
 const portal = new Xdp.Portal();
 const parent = XdpGtk.parent_new_gtk(workbench.window);
 const output = workbench.builder.get_object<Gtk.Picture>("output");
-const button = workbench.builder.get_object("button");
+const button = workbench.builder.get_object<Gtk.Button>("button");
 
 button.connect("clicked", () => {
   startScreencastSession().catch(console.error);