Social Networking :: Snippets Facebook API / Code Value Is NULL For Second Login

In this example ... Im getting error once im logged in no issue ... after that i tryed another login id the previous id details shown in that Lable ... 

The exact place for that error ...

FaceBookConnect.API_Key = "app key"; FaceBookConnect.API_Secret = "app secret code";
if (!IsPostBack) { if (Request.QueryString["error"] == "access_denied")
{ ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('User has denied access.')", true); return; }
string code = Request.QueryString["code"];  // here code takes Null for 2nd login if (!string.IsNullOrEmpty(code))
{ string data = FaceBookConnect.Fetch(code, "me"); Facebooklogin faceBookUser = new JavaScriptSerializer().Deserialize<Facebooklogin>(data); faceBookUser.PictureUrl = string.Format("https://graph.facebook.com/{0}/picture", faceBookUser.Id);