Object reference is set to instance of object

protected void listPOItems_OnItemEditing(object sender, System.Web.UI.WebControls.ListViewEditEventArgs e)
 {
     listPOItems.EditIndex = e.NewEditIndex;
     qcpackingitemscollections objItems = new qcpackingitemscollections();
     if (Session["StoresIndentItemCollection"] != null)
         objItems = (qcpackingitemscollections)Session["StoresIndentItemCollection"];

     listPOItems.DataSource = objItems;
     listPOItems.DataBind();
   //  BindEditDDLs();
   //  DropDownList ddlMate = (DropDownList)listIndentItems.Items[e.NewEditIndex].FindControl("ddlMaterials");
     TextBox txtseno = (TextBox)listPOItems.Items[e.NewEditIndex].FindControl("txtseno");
     List<PackingInstructionItem> objIndentItemCollection = new List<PackingInstructionItem>();
     if (Session["StoresIndentItems"] != null)
         objIndentItemCollection = (List<PackingInstructionItem>)Session["StoresIndentItems"];

     var selVal = objIndentItemCollection.Where(i => i.ItemId == e.NewEditIndex + 1).Single();
// The below line Object reference is set to instance of object error will occured//
     txtseno.Text =Convert.ToString( selVal.SerialNo);

    ScriptManager.GetCurrent(this).SetFocus(txtseno);
 }