Пустая ссылка исключение во время входа в систему с Facebook, используя Xamarin.авт в Xamarin.прошивкой

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Hi All,

i've implemented FB login using xamarin.auth in my xamarin forms app, which works fine in android, but in ios it raises NULL REFERENFCE EXCEPTION.

here is my class in PCL

public class FBLogin : ContentPage
{
public FBLogin()
{
//Page is Rendered in CustomRenderer for IOS and Android
}
public static async Task LoginSuccess()
{
App.Current.MainPage = new NavigationPage(new HomeScreen())
{
BarBackgroundColor = Color.FromHex("#009AD6"),
BarTextColor = Color.White,
};
}

This is how i wrote custom renderer in ios

public class FBLoginRenderer : PageRenderer
{
public override void ViewDidAppear(bool animated)
{
try
{
base.ViewDidAppear(animated);
var auth = new OAuth2Authenticator(
clientId: OAuthSettings.AppID, 
scope: OAuthSettings.Scope, 
authorizeUrl: new Uri(OAuthSettings.AutherizeURL), 
redirectUrl: new Uri(OAuthSettings.RedirectURL));
auth.Completed += async (sender, eventArgs) =>
{
DismissViewController(true, null);
if (eventArgs.IsAuthenticated)
{
await FBLogin.LoginSuccess();
}
else
{
NavigationController.PopViewController(false);
}
};
PresentViewController(auth.GetUI(), true, null);
}
catch (Exception ex)
{
}
}
}

can someone help me in solving this issue, i've implemented the same way of code in a sample project which worked fine but not working when i integrate to my project.

Output : after FB login app redirects to my desired screen then NULL REFERENFCE EXCEPTION. error occurs at the end of auth.completed event.

Что я уже пробовал:

Я добавил url-адрес успеха facebook '[^] 'в приложение facebook

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