I can’t seem to find a way to create menubar and a simple button.

With my current code, it will spawn two windows at the same time – one is just one big button and the other one is a file menu.

let window = gtk::ApplicationWindow::new(app);
window.set_title("First GTK Program");

let button = gtk::Button::with_label("Click me!");
// initialize the button

let menubar = {
    let file_menu = {
        let about_menu_item = gio::MenuItem::new(Some("About"), Some("app.about"));
        let quit_menu_item = gio::MenuItem::new(Some("Quit"), Some("app.quit"));

        let file_menu = gio::Menu::new();
        file_menu.append_item(&about_menu_item);
        file_menu.append_item(&quit_menu_item);
        file_menu
    };

    let menubar = gio::Menu::new();
    menubar.append_submenu(Some("File"), &file_menu);

    menubar
};

app.set_menubar(Some(&menubar));
app.set_child(Some(&button));

I think it is somewhere here that I do not set it up correctly. Should I use app.set_child? Or do I have to use hbox?