Problem with Map in Xamarin Forms Aplication.

I'm developing an app that must show a map with a pin on the position of one given place (lat and lon)
By the moment, I only want to open a map in a new window, after pusshing a button.
I have one simple code but the app crashes before open. Could anybody see if there is any mistake on this code?

using System;
using Xamarin.Forms;
using Xamarin.Forms.Maps;

namespace EventosXamarin
{
    public class PruebaPage : ContentPage
    {
        Map map;
        public PruebaPage ()
        {
             map = new Map() {
                IsShowingUser = true,
                HeightRequest = 100,
                WidthRequest = 960,
                VerticalOptions = LayoutOptions.FillAndExpand
            };
            var stack = new StackLayout { Spacing = 0 };
            stack.Children.Add(map);
            Content = stack;

        }
    }
}

And in the MainActivity on the android project I've also added the Maps Init;

    protected override void OnCreate (Bundle bundle)
        {
            base.OnCreate (bundle);

            global::Xamarin.Forms.Forms.Init (this, bundle);
            global::Xamarin.FormsMaps.Init(this, bundle);
            LoadApplication (new App ());
        }
    }

I have an Android Google maps API key too, but I think this is not the problem, because the app is not opening the Page.
Do I'm missing anything?

Thanks.