Object reference not set to an instance of an object

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C#3.5

Please help me how to solve this problem "Object reference not set to an instance of an object."

I have a datagrid & above error is generated in following code

protected void Bill_ItemCommand(object source, DataGridCommandEventArgs e)
    {
       // CheckBox ck = new CheckBox();
        CheckBox ck = (CheckBox)e.Item.FindControl("chk");
        string upval;
        if (ck.Checked)
            upval = "1";
        else
            upval = "0";

        if (e.CommandName == "Update")
        {
            OleDbConnection con = new OleDbConnection(@"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + Server.MapPath("~/App_Data/") + "Bills_db.mdb;Persist Security Info=False");


            // string strSql = @"Insert into January (Due_Date) values (txtDue_Date.txt)";

            string strSql = "update  tblBills set Remarks=" + upval + " where ID=" + (Bill.DataKeys[e.Item.ItemIndex]);
            con.Open();
            OleDbCommand cmd = new OleDbCommand(strSql, con);
            // cmd.Parameters.AddWithValue("@Name", name);



            cmd.ExecuteNonQuery();


            con.Close();
            BindData();

        }

I hav a check box in datagrid. When I view any page except the 1st page(I have 6 page in all) in datagrid that time it throws the above error. How to solve it? Please help me.