null exception is occur

Question

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 ;

worksheet = workbook.Sheets["sheet1"];

worksheet = workbook.ActiveSheet;

worksheet.Name = "Leave_Details";

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; i++)

{

for (int j = 0; j < dataGridView1.Columns.Count; j++)

{

worksheet.Cells[i + 2, j + 1] = dataGridView1.Rows[i].Cells[j].Value.ToString();

}

}

var saveFileDialoge = new SaveFileDialog();

saveFileDialoge.FileName = "Leave_Details";

saveFileDialoge.DefaultExt = ".xlsx";

if(saveFileDialoge.ShowDialog()==DialogResult.OK)

{

workbook.SaveAs(saveFileDialoge.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);

}

}