Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 3501:            string sql = "select id from info_class where fatherId=" + ids;
Line 3502:            OleDbDataReader dr = con.DrSelect(sql, null);
Line 3503:            while (dr.Read()) {
Line 3504:                columnids += "," + dr["id"].ToString();
Line 3505:                columnids += GetColumnIds_(dr["id"].ToString(), con);

Source File: d:\website\www.aitg.cn2\App_Code\jss_fc\JSSMethods.cs    Line: 3503

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.]
   jss_fc.JSSMethods.GetColumnIds(String ids, OperateDB con) in d:\website\www.aitg.cn2\App_Code\jss_fc\JSSMethods.cs:3503
   news.Pageing(String pagecount, String pagecurrent, String classId, String pageurl, String sortstr, String parameters) in d:\website\www.aitg.cn2\news.aspx.cs:144
   news.Page_Load(Object sender, EventArgs e) in d:\website\www.aitg.cn2\news.aspx.cs:107
   System.Web.Util.CalliEventHandlerDelegateProxy.Callback(Object sender, EventArgs e) +85
   System.Web.UI.Control.OnLoad(EventArgs e) +74
   System.Web.UI.Control.LoadRecursive() +120
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2847

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.0.30319.17929