ToSignIn.swift

AuthenticationManager.instance.getTokenInteractively(parentView: self) {
    (token: String?, error: Error?) in
    DispatchQueue.main.async {
        guard let _ = token, error == nil else {
            // Show the error and stay on the sign-in page
            let alert = UIAlertController(title: "Error signing in",
                                          message: error.debugDescription,
                                          preferredStyle: .alert)

            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
            self.present(alert, animated: true)
            return
        }
        // Signed in successfully
        // Go to welcome page
}