private async void _loginUser(object parameter)
        {
            var passwordBox = parameter as PasswordBox;
            var password    = passwordBox.Password;

            if (await _service.LoginUser(_userName, password))
            {
                UserPage userPage = new UserPage();
                MainWindow.MainInstance.NavigationFrame.Navigate(userPage);
            }
            else
            {
                MessageBox.Show("Invalid user name or password.");
                return;
            }
        }