Xamarin Forms - Draw polyline for WinPhone 8.1 part

I cannot find the way to draw a line (polyline) with my CustomMapRenderer.cs..

var polyline = new MapPolyline();
polyline.StrokeColor = Windows.UI.Color.FromArgb(128, 255, 0, 0);
polyline.StrokeThickness = 5;
polyline.Path = new Geopath(coordinates);
nativeMap.MapElements.Add(polyline);

An exception of type 'System.Exception' occurred in CaptainSam.WinPhone.ni.EXE but was not handled in user code. Additional information: Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED))

The exception comes from polyline.Path = new Geopath(coordinates);

It's Highlight a Route on a Map of Microsoft tutorial, so what's happens?

I know they says it's an example for UWP, but I also tried and it also throws the same exception...

After a long time, I found why it didn't works...

If you're doing like me, if you do a HttpRequest to something as Google Direction API, then the result will comes after the first passage in the OnElementChanged().

Because formsMap.RouteCoordinates isn't null but empty, the Catastrophic failure (Exception from HRESULT: 0x8000FFFF (E_UNEXPECTED)) is thrown..

This is the good CustomMapRenderer for PolyLine use

using PROJECT;
using PROJECT.UWP;
using System.Collections.Generic;
using Windows.Devices.Geolocation;
using Windows.UI.Xaml.Controls.Maps;
using Xamarin.Forms.Maps;
using Xamarin.Forms.Maps.UWP;
using Xamarin.Forms.Platform.UWP;

[assembly: ExportRenderer(typeof(CustomMap), typeof(CustomMapRenderer))]
namespace PROJECT.UWP
{
    public class CustomMapRenderer : MapRenderer
    {
        MapControl nativeMap;
        CustomMap formsMap;

        protected override void OnElementChanged(ElementChangedEventArgs<Map> e)
        {
            base.OnElementChanged(e);

            if (e.OldElement != null)
            {
                nativeMap = Control as MapControl;
            }

            if (e.NewElement != null)
            {
                formsMap = (CustomMap)e.NewElement;
                nativeMap = Control as MapControl;
                UpdatePolyLine();
            }
        }

        protected override void OnElementPropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged(sender, e);
            if (this.Element == null || this.Control == null)
                return;

            if (e.PropertyName == CustomMap.RouteCoordinatesProperty.PropertyName)
            {
                UpdatePolyLine();
            }
        }

        private void UpdatePolyLine()
        {
            if (formsMap != null && formsMap.RouteCoordinates.Count > 0)
            {
                List<BasicGeoposition> coordinates = new List<BasicGeoposition>();

                foreach (var position in formsMap.RouteCoordinates)
                {
                    coordinates.Add(new BasicGeoposition() { Latitude = position.Latitude, Longitude = position.Longitude });
                }

                Geopath path = new Geopath(coordinates);
                MapPolyline polyline = new MapPolyline();
                polyline.StrokeColor = Windows.UI.Color.FromArgb(128, 255, 0, 0);
                polyline.StrokeThickness = 5;
                polyline.Path = path;
                nativeMap.MapElements.Add(polyline);
            }
        }
    }
}