Navigating from Android activity back to forms

I have forms project where I use a custom ContentPage and a PageRenderer based on this code:

forums.xamarin.com/discussion/18014/mixing-forms-content-and-native-activity

I can navigate to my android activity with :

buttonB.Clicked += async (object sender, EventArgs e) => 
                await Navigation.PushAsync(new MapPage());

but I cant figure out how to navigate back to the forms page, the back button does not do anything.

MapPage is simply and empty ContentPage.

My Renderer looks like this:

`
using System;
using Xamarin.Forms.Platform.Android;
using Android.Gms.Maps;
using Xamarin.Forms;
using Android.App;
using Android.Views;
using Android.Gms.Maps.Model;
using System.Collections.Generic;
using System.Threading.Tasks;
using System.ComponentModel;
using Android.Graphics;

[assembly:ExportRenderer(typeof(FormsMaps.MapPage), typeof(FormsMaps.Droid.MapPageRenderer))]
namespace FormsMaps.Droid
{
public class MapPageRenderer : PageRenderer
{
Android.Views.View view;
private MapFragment _mapFragment;
private GoogleMap _map;
private bool _gettingMap;
private MapPage _mapPage;

    private Activity _activity;

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

        _mapPage = (MapPage)e.NewElement;
        SetPage(_mapPage);
        // load the axml file for this view
        _activity= this.Context as Activity;
        _activity.SetContentView(Resource.Layout.MapLayout);

        view = _activity.LayoutInflater.Inflate(Resource.Layout.MapLayout, this, false);
        AddView(view);


        InitMapFragment();
        SetupMapIfNeeded(); // It's not gauranteed that the map will be available at this point.
    }

    private void InitMapFragment()
    {

        _mapFragment = _activity.FragmentManager.FindFragmentByTag("map") as MapFragment;
        if (_mapFragment == null)
        {
            GoogleMapOptions mapOptions = new GoogleMapOptions()
                .InvokeMapType(GoogleMap.MapTypeTerrain)
                .InvokeZoomControlsEnabled(true)
                .InvokeCompassEnabled(true)
                .InvokeRotateGesturesEnabled(true)
                .InvokeScrollGesturesEnabled(true)
                .InvokeTiltGesturesEnabled(true)
                .InvokeZoomGesturesEnabled(true);


            FragmentTransaction fragTx = _activity.FragmentManager.BeginTransaction();
            _mapFragment = MapFragment.NewInstance(mapOptions);
            fragTx.Add(Resource.Id.map, _mapFragment, "map");
            fragTx.Commit();
            Markers = new List<Marker>();
        }
    }



    private void SetupMapIfNeeded()
    {
        if(null != _map || _gettingMap) return;

        var mapReadyCallback = new MyOnMapReady();

        mapReadyCallback.MapReady += (sender, args) =>
        {
                _map = mapReadyCallback.Map;
                // set up annotations etc here

            _gettingMap = false;
        };

        _gettingMap = true;
        _mapFragment.GetMapAsync(mapReadyCallback); 

    }

}

}`

How can I navigate back?