Object reference not set to an instance of an object at Sanbox Solution

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C#

SharePoint

Hi Friend,
I am using sandbox web-part For exporting grid data to excel sheet.
I am getting "Object reference not set to an instance of an object" Error At
Response.End()

Below is my code:

Context.Response.Clear();
                   Context.Response.ContentType = "application/ms-excel";
                   Context.Response.ContentEncoding = System.Text.UnicodeEncoding.UTF8;
                   Context.Response.Charset = "UTF-8";
                   Context.Response.AddHeader("content-disposition", string.Format("attachment; filename=Download.xls"));

                   using (System.IO.StringWriter sw = new System.IO.StringWriter())
                   {
                       using (HtmlTextWriter htw = new HtmlTextWriter(sw))
                       {
                           ExcellExport.RenderControl(htw);
                           Context.Response.Write(htw.ToString());
                          // HttpContext.Current.Response.Write(htw.ToString());
                          Context.Response.End();
                           Context.Response.Flush();
                       }
                   }

Help Me Soon .
Thanks.