Object reference not set to an instance of an object????

public void ImportToStudent()

try

//Gets the filename, not the path

string filename = ofd.SafeFileName;

string connection = @"Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + "E:\\..\\PSAtechZTimetableSystem\\SpreadSheet.xls" + "; Extended Properties='Excel 8.0; IMEX=1; HDR=YES'";

// Here is the call to Open a Workbook in Excel

// It uses most of the default values (except for the read-only which we set to true)

Microsoft.Office.Interop.Excel.Workbook theWorkbook = ExcelObj.Workbooks.Open(ofd.FileName, 0, true, 5,

"", "", true, Microsoft.Office.Interop.Excel.XlPlatform.xlWindows,"\t", false, false, 0, true, true, true);

// get the collection of sheets in the workbook

Microsoft.Office.Interop.Excel.Sheets sheets = theWorkbook.Worksheets;

// get the first and only worksheet from the collection of worksheets

Microsoft.Office.Interop.Excel.Worksheet worksheet = (Microsoft.Office.Interop.Excel.Worksheet)sheets.get_Item(1);

using (SqlConnection conPSAtechZ = new SqlConnection(connection))

using (SqlCommand commandExcel = conPSAtechZ.CreateCommand())

System.Data.DataTable table = new System.Data.DataTable();

commandExcel.CommandText = "SELECT [StudentID] AS [StudentID]," + "[ModuleCode] AS [ModuleCode], " + "FROM [Sheet1$]";

//Open the Excel Connection

using (SqlDataReader dr = commandExcel.ExecuteReader(CommandBehavior.CloseConnection))

while (dr.Read() && dr.HasRows)

lblResults.Text = "Importing...";

try

studentdal.RegisterStudents(new RegisterStudent(Convert.ToString(dr["StudentID"]), Convert.ToString(dr["ModuleCode"])));

commandExcel.CommandType = CommandType.StoredProcedure;

lblResults.Text = "Successfully Imported!!";

catch (Exception Ex)

MessageBox.Show("Error!!\n " + Ex.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);

catch (Exception EX)

MessageBox.Show("The spreadsheet is not in the correct format!\n\n" + EX.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);