object reference error

protected void btnsort_Click(object sender, EventArgs e)
   {
       String expression = GrdATDataView.Columns[0].HeaderText;
       String Dire = "";
       SortDirection direction;


       switch (DropDownList1.SelectedValue.ToString().Trim())
       {
           case "asc":
               direction = SortDirection.Ascending;
               Dire = " Asc";
               break;
           case "des":
               direction = SortDirection.Descending;
               Dire = " Desc";
               break;
           default:
               direction = SortDirection.Ascending;
               break;
       }

  DT.DefaultView.Sort = "PRODUCT_GROUP_DESC " + Dire; //here this error.
  GrdATDataView.DataSource = DT;
  GrdATDataView.DataBind();
   }