Silverlight web Project中的Twitter web服务

我目前正在使用这个页面http://www.silverlightshow.net/items/Silvester-A-Silverlight-Twitter-Widget.aspx来制作一个Silverlight应用程序，该应用程序可以获取用户的twitter feed并显示它。

我已经做了一些改变的代码，但它应该仍然工作相同。但是，当我尝试转换xml:

时，我收到此错误消息。

>System.NullReferenceException was unhandled by user code
  Message=Object reference not set to an instance of an object.
  Source=USElab.Web
  StackTrace:
       at USElab.Web.TwitterWebService.<GetUserTimeline>b__1(XElement status) in H:'USE'USE'USE'USElab'USElab.Web'TwitterWebService.asmx.cs:line 58
       at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
       at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
       at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
       at USElab.Web.TwitterWebService.GetUserTimeline(String twitterUser, String userName, String password) in H:'USE'USE'USE'USElab'USElab.Web'TwitterWebService.asmx.cs:line 57
  InnerException: 

我不明白为什么会发生这种事。下面是代码:

   namespace USElab.Web
   {
/// <summary>
/// Summary description for TwitterWebService
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class TwitterWebService : System.Web.Services.WebService
{
    private const string UserTimelineUri = "http://twitter.com/statuses/user_timeline/{0}.xml";
    private const string StatusElementName = "status";
    private const string CreatedAtElementName = "created_at";
    private const string TextElementName = "text";
    private const string ProfileImageUrlElementName = "profile_image_url";
    private const string NameElementName = "name";
    private const string UserElementName = "user";
    private const string UserUrlElementName = "url";
    public const int RequestRateLimit = 70;
    [WebMethod]
        public List<TwitterItem> GetUserTimeline( string twitterUser, string userName, string password )
        {
              if ( string.IsNullOrEmpty( twitterUser ) )
              {
                  throw new ArgumentNullException( "twitterUser", "twitterUser parameter is mandatory" );
              }
              WebRequest rq = HttpWebRequest.Create( string.Format( UserTimelineUri, twitterUser ) );
              if ( !string.IsNullOrEmpty( userName ) && !string.IsNullOrEmpty( password ) )
              {
                  rq.Credentials = new NetworkCredential( userName, password );
              }
              HttpWebResponse res = rq.GetResponse() as HttpWebResponse;
              // rate limit exceeded
              if ( res.StatusCode == HttpStatusCode.BadRequest )
              {
                  throw new ApplicationException( "Rate limit exceeded" );
              }
              XDocument xmlData = XDocument.Load(XmlReader.Create(res.GetResponseStream()));
              List<TwitterItem> data = (from status in xmlData.Descendants(StatusElementName)
                           select new TwitterItem
                           {
                               Message = status.Element(StatusElementName).Element(TextElementName).Value.Trim(),
                               ImageSource = status.Element(StatusElementName).Element(ProfileImageUrlElementName).Value.Trim(),
                               UserName = status.Element(StatusElementName).Element(UserElementName).Value.Trim()
                           }).ToList();

                return data;
          }

根据调试器，这发生在开始select new TwitterItem的那一行。

任何帮助都将非常感激!