IListSource does not contain any data sources

Question

i face this error during runtime of my website in asp.net and C#. plz chk my code and reply
using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
using System.Data.SqlTypes;
 
public partial class Default2 : System.Web.UI.Page
{
SqlConnection cnx = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename=|DataDirectory|Coupon Management.mdf;Integrated Security=True;Connect Timeout=30;User Instance=True");
SqlDataAdapter da1 = new SqlDataAdapter();
DataSet ds = new DataSet();
SqlCommand cmd = new SqlCommand();
DataTable dataTable;
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
FillVendorGrid();
}
}
private void FillVendorGrid()
{
dataTable = new DataTable();
cmd.Connection = cnx;
 
cmd.CommandText = "SELECT Coupon_Info.RecievingDate, CouponType.CouponName, Coupon_Info.Serial_Start, Coupon_Info.Serial_End, Coupon_Info.Recieved_By FROM Coupon_Info INNER JOIN CouponType ON Coupon_Info.Coupon_TypeID = CouponType.CouponTypeID";
da1 = new SqlDataAdapter(cmd);
da1.Fill(dataTable);
GridView1.DataSource = dataTable;
GridView1.DataSource = ds;
GridView1.DataSourceID = String.Empty;
//GridView1.DataBind();
// GridView1.DataSource = ds;
// GridView1.DataBind();
}
protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
GridView1.PageIndex = e.NewPageIndex;
FillVendorGrid();
}
protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
cmd.Connection = cnx;
cmd.CommandText = "DELETE FROM Coupon_Info WHERE RecievingDate='" + GridView1.DataKeys[e.RowIndex].Values[0].ToString() + "'";
cnx.Open();
cmd.ExecuteNonQuery();
cnx.Close();
FillVendorGrid();
}
}