This is what I am doing:

DataSet ds = SqlHelper.ExecuteDataset(SPString, CommandType.StoredProcedure, "sp_name", parameters);
dgdownload.DataSource = ds.Tables[0];
dgdownload.DataBind();


// Set the content type to Excel.
Response.ContentType = "application/vnd.ms-excel";


// Remove the charset from the Content-Type header.
Response.Charset = "";
	            

// Turn off the view state.
Page.EnableViewState = false;


// Get the HTML for the control.
dgdownload.RenderControl(new HtmlTextWriter(tw));
tw.Flush();
tw.Close();

I got this code from many threads from this forum but I am having a problem. The file downloads fine but at the end of file I get this error

The Following Exception occured. Exception : System.NullReferenceException: Object 
reference not set to an instance of an object. at 
System.Web.UI.HtmlTextWriter.RenderBeginTag(HtmlTextWriterTag tagKey) at 
System.Web.UI.WebControls.WebControl.RenderBeginTag(HtmlTextWriter writer) at 
System.Web.UI.WebControls.WebControl.Render(HtmlTextWriter writer) at 
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) at 
System.Web.UI.Control.RenderChildren(HtmlTextWriter writer) at 
System.Web.UI.WebControls.WebControl.RenderContents(HtmlTextWriter writer) at 
System.Web.UI.WebControls.BaseDataList.Render(HtmlTextWriter writer) at 
System.Web.UI.Control.RenderControl(HtmlTextWriter writer) at 
Speedpay.download.Page_Load(Object sender, EventArgs e) in 
c:\appsdev\speedpayrpt\speedpay\download.aspx.cs:line 91 Source : System.Web 	

How do I get rid of this error? thanks,