MapView does not show

I'm building a xamarin forms using PCL. One of my button should redirect to a map.

To do that, in the ios project, I have the following code

var map = new MKMapView(UIScreen.MainScreen.Bounds)
            {
                //ShowsUserLocation = true
            };
            var mkPointAnnotation = new MKPointAnnotation
            {
                Title = title,
            };
            mkPointAnnotation.SetCoordinate(new CLLocationCoordinate2D(42.364260, -71.120824));
            map.AddAnnotation(mkPointAnnotation);
            var contentPage = new ContentPage();
            contentPage.Title = title;
            var uiViewController = contentPage.CreateViewController();
            uiViewController.View.Frame = UIScreen.MainScreen.Bounds;
            uiViewController.View = map;
            UIApplication.SharedApplication.KeyWindow.RootViewController.PresentViewController(uiViewController, true, null);

However, when I execute this code, I have a blank page.

I found on the forum (http://forums.xamarin.com/discussion/8418/mkmapview-not-showing-ios-7) that I can add some compilation arguments "-gcc_flags="-framework CoreGraphics"".
However, my application does not launch as I suspect a problem with System.Net.Http. I have the following warning :
The existing binding redirect on "System.Net.Http, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a" is incorrect. Consider removing it from the application configuration file. The build will replace it with: "<bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0" xmlns="urn:schemas-microsoft-com:asm.v1" />".

To summarize, on hand, my application launches but my map is a blank page. On the other hand my app crashes when (maybe) it retrieves data from the internet.

Can somebody tell what's wrong with my code or my app?

Thanks