ProgressRing is not shown

I need to show progressring while running a task, but it does not appear and the screen is frozen until it runs.

        private async void btn_Criar_Click(object sender, RoutedEventArgs e)
    {
        pgr_Carregando.IsActive = true;
        if (Fitas == null)
            Fitas = new ObservableCollection<Fitas>();

        await Set_Listas();
        pgr_Carregando.IsActive = false;
        this.Frame.Navigate(typeof(MainPage), Fitas);
    }

    private Task Set_Listas()
    {
        string path = Path.Combine(Windows.ApplicationModel.Package.Current.InstalledLocation.Path, @"Assets\bd.db");
        Fitas = SG_Classes.Fitas.Popular_Fitas(path);
        return Task.CompletedTask;
    }