Xamarin android facebook auth NullReferenceException

Question

我在尝试通过Facebook验证我的应用时遇到问题。在Facebook表单中输入用户名和密码后，我抓到了NullReferenceException。 我注意到只有在使用Android 6.0进行编译时才会发生这种情况。

[assembly: ExportRenderer(typeof(FirstPage), typeof(FirstPageRenderer))]
namespace Dating.Droid.Renderers
{
public class FirstPageRenderer : PageRenderer
{
    public FirstPageRenderer()
        {
            var activity = this.Context as Activity;
            if (activity != null)
            {
                var auth = new OAuth2Authenticator(
                    clientId: "MyFacebookId", // your OAuth2 client id
                    scope: "", // the scopes for the particular API you're accessing, delimited by "+" symbols
                    authorizeUrl: new Uri("https://m.facebook.com/dialog/oauth/"),
                    redirectUrl: new Uri("http://www.facebook.com/connect/login_success.html"));

                auth.Completed += (sender, eventArgs) =>
                {
                    if (eventArgs.IsAuthenticated)
                    {
                        // Authenticated
                    }
                    else
                    {
                        // The user cancelled
                    }
                };

                activity.StartActivity(auth.GetUI(activity));
            }
        }
    }
}

Exception appears on auth.Completed event

Stack Trace：在D：.Dv \ Real中的Dating.Droid.Renderers.FirstPageRenderer +＆lt;＆gt; c。＆lt; .ctor＆gt; b__0_0（System.Object sender，Xamarin.Auth.AuthenticatorCompletedEventArgs eventArgs）[0x00002] \ Xamarin \ Dating \ Dating.Droid \ Renderers \ FirstPageRenderer.cs：31