);         }          /// <summary>         /// Shows an error message to the user and allows a callback operation when the message is completed.         /// </summary>         /// <param name="message">The message.</param>         /// <param name="caption">The caption.</param>         /// <exception cref="ArgumentException">The <paramref name="message"/> is <c>null</c> or whitespace.</exception>         public virtual Task<MessageResult> ShowErrorAsync(string message, string caption = "")         {             if (string.IsNullOrEmpty(caption))             {                 caption = Catel.ResourceHelper.GetString("