The Exception is throwed as follows:
Activity xamarin.auth.WebAuthenticatorActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@406e70a0 that was originally added here android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@40662fc0 is not valid; is your activity running?
Work flow of Xamarin.Auth is as follows (I debugged inside the Xamarin.Auth source code): 1 -> You create an instance of Oauth2Authenticator, that will create an activty with a Webview inside. The Webview will open the Facebook login page. 2 -> You type login information and press Login. 3 -> You manually handle Completed event which fired by Xamarin.Auth if the login information was correct. 4 -> Xamarin.Auth finishs the activity. 5 -> Xamarin.Auth make a request to redirect_uri which was passed to Oauth2Authenticator.
As everyone can see, the step 4 & 5 is crazy. The author of Xamarin.Auth should fix this problem.