Xamarin ASWebAuthenticationSession iOS 13

fix.cs

// Source https://github.com/auth0/auth0-oidc-client-net/blob/master/src/Auth0.OidcClient.iOS/ASWebAuthenticationSessionBrowser.cs#L22-L32

ASWebAuthenticationSession asWebAuthenticationSession = null;

asWebAuthenticationSession = new ASWebAuthenticationSession(
    new NSUrl(options.StartUrl),
    options.EndUrl,
    (callbackUrl, error) =>
    {
        tcs.SetResult(CreateBrowserResult(callbackUrl, error));
        asWebAuthenticationSession.Dispose();
    });
#if __IOS__13
  asWebAuthenticationSession.PresentationContextProvider = UIApplication.SharedApplication.KeyWindow
#endif
asWebAuthenticationSession.Start();