Here Im Getting Data From Sql To Gridview And Export To Excele .. It's Not Working I Dont Know Wats Wrong With This Coding

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Text.RegularExpressions;
using Satham.DataAccess1;

namespace exptoexl
{
    public partial class Form1 : Form
    {
        DataTable dt = new DataTable();
        DBAccess db = new DBAccess();
        public Form1()
        {
            InitializeComponent();
        }

        private void btnload_Click(object sender, EventArgs e)
        {
           // DataRow dr = dt.NewRow();
           // dt.Rows.Add(dr);
            dataGridView1.DataSource = dt;
            string sSql = "SELECT * FROM `fxwalkincustomer`";
            db.ExecuteNonQuery(sSql);
            dt = db.GetDataTable(sSql);
            dataGridView1.DataSource = dt;
            MessageBox.Show("Loading Complete");

        }

        private void btnexport_Click(object sender, EventArgs e)
        {
            load.InitialDirectory = "C:/Windows";
            load.Title = "save as ExcelFile";
            load.FileName = "";
            load.Filter = "Excel Files(2003)|*.xls|Excel Files(2007)|*.xlsx|Excel Workbook|*.xlsx";

            if (load.ShowDialog() != DialogResult.Cancel)
            {

                Microsoft.Office.Interop.Excel._Application app = new Microsoft.Office.Interop.Excel.Application();
                Microsoft.Office.Interop.Excel._Workbook workbook = app.Workbooks.Add(Type.Missing);
                Microsoft.Office.Interop.Excel._Worksheet worksheet = null;
                app.Visible = false;
                worksheet = workbook.Sheets["Sheet1"];
                worksheet = workbook.ActiveSheet;
                app.Columns.ColumnWidth = 20;
                worksheet.Name = "Exported from dtaaGridView1";
                             

                for (int i = 1; i < dataGridView1.Columns.Count + 1; i++)
                {
                    worksheet.Cells[1, i] = dataGridView1.Columns[i - 1].HeaderText;
                }
                for (int i = 0; i < dataGridView1.Rows.Count - 1; i++)
                {
                    for (int j = 0; j < dataGridView1.Columns.Count; j++)
                    {
                        worksheet.Cells[i + 2, j + 1] = dataGridView1.Rows[i].Cells[j].Value.ToString();
                    }
                }
                app.ActiveWorkbook.SaveCopyAs(load.FileName.ToString());
                app.ActiveWorkbook.Saved = true;
                app.Quit();
            }
        }
    }
}