ช่วยดูให้หน่อยครับว่ามันผิดตรงไหนขึ้น error Object reference not set to an instance of an object.

        private void genFile(string coopnumber)
        {
            string Date1 = dateTimePicker1.Value.ToString("ddMM");
            StreamWriter writer = new StreamWriter("C:\\ktbank_file\\out\\KTB" + coopnumber + "" + Date1 + ".txt", false, Encoding.Default);

            for (int i = 0; i < dataGridView1.Rows.Count; i++)
            {
                writer.Write(dataGridView1.Rows[i].Cells[0].Value.ToString().Substring(0, 1));
                writer.Write(dataGridView1.Rows[i].Cells[1].Value.ToString().PadRight(4, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[2].Value.ToString().PadRight(19, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[3].Value.ToString().PadRight(4, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[4].Value.ToString().PadRight(20, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[5].Value.ToString().PadRight(16, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[6].Value.ToString().PadRight(1, ' '));
                writer.Write(dataGridView1.Rows[i].Cells[7].Value.ToString().PadRight(35, ' '));
                writer.WriteLine();
            }
            MessageBox.Show("Convert to txt sucsses", "Sucsses");
            writer.Close();

            dataGridView1.DataSource = null;
            dataGridView1.Refresh();
        }