Launch an Android intent from the Android Project into the shared Project

Hi,

I have a portable Project and im trying to launch an Android Activity out of the Android Project and put it into the shared Project but I always get an unhandled exception.

In my Portable Project i have an Interface IOAuth2 and a ContentPage:

namespace Test.Services
 {   public interface IOAuth2
    {
        void Login(bool allowCancel);
    }
}

namespace Test.Pages
{
    public partial class LogIn : ContentPage
     {
public LogIn{
InitializeComponent();
DependencyService.Get<IOAuth2>().Login(true);
}
}
}

And in the Android Project I have this:
[assembly: Xamarin.Forms.Dependency(typeof(Test.Droid.Services.OAuth2))]
`namespace Test.Droid.Services
{
[Activity(Label = "Test.Droid.Services")]

public class OAuth2 : Activity , IOAuth2
{
private OAuth2Authenticator _auth;
public void Login(bool allowCancel)
{

        _auth = new OAuth2Authenticator(
            clientId:".......apps.googleusercontent.com",
            //clientSecret: "......",
            scope: "https://www.googleapis.com/auth/userinfo.email",
            authorizeUrl: new Uri("https://accounts.google.com/o/oauth2/auth"),
            redirectUrl: new Uri("http://localhost:8080/")  //new Uri("urn:ietf:wg:oauth:2.0:oob")                     
            );   
    _auth.AllowCancel = allowCancel;


        _auth.Completed += (sender, ee) =>
        {
            if (!ee.IsAuthenticated)
            {


                return;
            }


        };
            var intent = _auth.GetUI(this);
             StartActivity(intent);}}`

I always get an unhanlded exception in this line var intent = _auth.GetUI(this);

Can anyone help me?

Thank you very much!!