SkiaSharp:SKSurface.Create returns alway null

Hi

I want to create my own simple Xamaran.Forms SkiaSharp example based on https://github.com/mono/SkiaSharp/tree/master/samples (which works)

PCL Main Page :

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:local="clr-namespace:TestSkia"
             x:Class="TestSkia.MainPage">

 <Grid>
   <local:SkiaView x:Name="SkiaView"></local:SkiaView>
 </Grid>

</ContentPage>

PCL Skia View

namespace TestSkia
{
    public class SkiaView : View
    {


    }
}

The Android SkiaView and the SkiaViewRenderer

using System;
using Android.Views;
using Android.Graphics;
using SkiaSharp;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer(typeof(TestSkia.SkiaView), typeof(TestSkia.Droid.SkiaViewRenderer))]

namespace TestSkia.Droid
{
    public class SikaView : Android.Views.View
    {
        readonly SkiaView skiaView;
        private Bitmap bitmap;
        public SikaView(Android.Content.Context context, SkiaView skiaView) : base(context)
        {
            this.skiaView = skiaView;
        }

        protected override void OnDraw(Android.Graphics.Canvas canvas)
        {
            base.OnDraw(canvas);

            if (bitmap == null || bitmap.Width != canvas.Width || bitmap.Height != canvas.Height)
            {
                if (bitmap != null)
                    bitmap.Dispose();

                bitmap = Bitmap.CreateBitmap(canvas.Width, canvas.Height, Bitmap.Config.Argb8888);
            }

            try
            {
                using (var surface = SKSurface.Create(canvas.Width, canvas.Height, SKImageInfo.PlatformColorType, SKAlphaType.Premul, bitmap.LockPixels(), canvas.Width * 4))
                {
                    var skcanvas = surface.Canvas;
                    skcanvas.Scale(((float)canvas.Width) / (float)skiaView.Width, ((float)canvas.Height) / (float)skiaView.Height);
                    skcanvas.DrawCircle(100, 100, 100, new SKPaint() { Color = new SKColor(155, 155, 155) });

                }
            }

            finally
            {
               // bitmap.UnlockPixels();
            }

        }

    }

    public class SkiaViewRenderer : ViewRenderer<SkiaView, TestSkia.Droid.SikaView>
    {
        protected override void OnElementChanged(ElementChangedEventArgs<SkiaView> e)
        {
            base.OnElementChanged(e);

            if (Control == null)
            {
                var view = new TestSkia.Droid.SikaView(Context, Element);
                SetNativeControl(view);
            }
        }
    }
}

SKSurface.Create returns always null, What I'am doing wrong ?

Thanks Wolfgang