کد های من کار میکردن اما الان خوب ی سری فیلد خالی دارم و دیگه انجام نمیشه
 

    worksheet.Cells[i + 2, j + 1] = radGridView2.Rows[i].Cells[j].Value.ToString();

 {"Object reference not set to an instance of an object."} تو این خط ارور میگیره 

این هم کد اصلی

object mis = System.Reflection.Missing.Value;
            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 = (Worksheet)workbook.Sheets["Sheet1"];
            worksheet = (Worksheet)workbook.ActiveSheet;
            worksheet.Name = "Export";


            for (int i = 1; i < radGridView2.Columns.Count + 1; i++)
            {
                worksheet.Cells[1, i] = radGridView2.Columns[i - 1].HeaderText;
            }
            for (int i = 0; i < radGridView2.Rows.Count; i++)
            {
                for (int j = 0; j < radGridView2.Columns.Count; j++)
                {
                    worksheet.Cells[i + 2, j + 1] = radGridView2.Rows[i].Cells[j].Value.ToString();
                }
            }

            SaveFileDialog sfd = new SaveFileDialog();
            sfd.Filter = "Excel Document(*.xlsx)|*.xlsx";
            sfd.FileName = "Export";
            if (sfd.ShowDialog() == DialogResult.OK)
            {
                workbook.SaveAs(sfd.FileName, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, Type.Missing, Type.Missing, Type.Missing, Type.Missing, Type.Missing);
            }
            app.Quit();