private void Login()
 {
     if (string.IsNullOrWhiteSpace(User.UserName) || string.IsNullOrWhiteSpace(User.Password))
     {
         MessageBox.Show("You must fill the username and password fields.");
     }
     else
     {
         if (_userManager.InvokeLogin(User))
         {
             _navigationService.NavigateTo("Contacts");
         }
     }
 }