title: Xamarin android facebook auth NullReferenceException
body: <p>I am faced with problem when trying to authenticate my app via facebook. After entering a username and password in the facebook form, I caught a <code>NullReferenceException</code>. 
  I've noticed that it happens only when I compile with Android 6.0.</p>
  
  <pre><code>[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) =&gt;
                  {
                      if (eventArgs.IsAuthenticated)
                      {
                          // Authenticated
                      }
                      else
                      {
                          // The user cancelled
                      }
                  };
  
                  activity.StartActivity(auth.GetUI(activity));
              }
          }
      }
  }
  </code></pre>
  
  <p><a href="http://i.stack.imgur.com/lHYO4.png" rel="nofollow">Exception appears on auth.Completed event</a></p>
  
  <p>Stack Trace:   at Dating.Droid.Renderers.FirstPageRenderer+&lt;>c.&lt;.ctor>b__0_0 (System.Object sender, Xamarin.Auth.AuthenticatorCompletedEventArgs eventArgs) [0x00002] in D:\Dev\Real\Xamarin\Dating\Dating.Droid\Renderers\FirstPageRenderer.cs:31 </p>