Xamarin Forms Labs Camera Issues

Hi guys, I already implement Xamarin.Forms.Labs into project and use the sample code to test. It doesn't show any error when i clicked the button, but it does not response anything. Please kindly help me. thank you.

*I already set the CAMERA-Permission, and the WRITE_EXTERNAL_STORAGE-Permission.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection.Emit;
using System.Text;
using System.Threading.Tasks;
using Xamarin.Forms;
using Xamarin.Forms.Labs;
using Xamarin.Forms.Labs.Services;
using Xamarin.Forms.Labs.Services.Media;

namespace eRestaurant.Pages
{
public class AddFoodPage : ContentPage
{
private ImageSource imageSource;
private IMediaPicker mediaPicker;
private Image img;
private string status;

public AddFoodPage()
{
    this.Title = "Camera Test";


    NavigationPage.SetHasNavigationBar(this, false);


    img = new Image() { HeightRequest = 300, WidthRequest = 300, BackgroundColor = Color.FromHex("#D6D6D2"), Aspect = Aspect.AspectFit };


    var addPictureButton = new Button()
    {
        Text = "Select Picture",
        Command = new Command(async () => { await SelectPicture();})
    };


    StackLayout stack = new StackLayout();
    stack.VerticalOptions = LayoutOptions.FillAndExpand;

    stack.Children.Add(new BoxView { Color = Color.Transparent, HeightRequest = 20 });
    stack.Children.Add(addPictureButton);
    stack.Children.Add(img);


    ScrollView scrollview = new ScrollView
    {
        Orientation = ScrollOrientation.Vertical,
        VerticalOptions = LayoutOptions.FillAndExpand,
        Content = stack

    };

    this.Content = new StackLayout
    {
        Children = { scrollview }
    };
}

private async Task SelectPicture()
{

    mediaPicker = DependencyService.Get<IMediaPicker>();

    imageSource = null;

    try
    {
        var mediaFile = await mediaPicker.SelectPhotoAsync(new CameraMediaStorageOptions
            {
                DefaultCamera = CameraDevice.Front,
                MaxPixelDimension = 400
            });
        imageSource = ImageSource.FromStream(() => mediaFile.Source);
        img.Source  = imageSource;
    }
    catch (System.Exception ex)
    {
        this.status = ex.Message;
    }
}

}