[Resolvido] Erro ao criar menu dinamica

Olá a todos, estou seguindo este tutorial:

http://jobs.aspalliance.com/822_Building_a...using_ASPNET_20

e este é meu código:

protected void Page_Load(object sender, EventArgs e)
{
if (!(IsPostBack))
{

 criaMenu();
 }
 }
 public void criaMenu()
 {
 DataSet ds = new DataSet();

 BLMenus objBLMenus = new BLMenus();
 ds = objBLMenus.listaMenus();

 ds.DataSetName = "Menus";
 ds.Tables[0].TableName = "Menu";
 DataRelation relation = new DataRelation("ParentChild",
 ds.Tables["Menu"].Columns["men_codigo"],
 ds.Tables["Menu"].Columns["men_parentcod"], true);

 relation.Nested = true;
 ds.Relations.Add(relation);
 if (ds.GetXml() != null)
 {
 xmlDataSource = ds.GetXml();
 }

 }

Mas ta dando erro de nullreference e eu não consigo achar, já destrichei o codigo, e não consigo achar, alguem pode me ajudar?

olha o erro abaixo:

Server Error in '/' Application.
Object reference not set to an instance of an object.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 43: objXMl.XPath = "MenuItems/MenuItem";
Line 44:

Line 45: menu.DataSource = objXMl;
Line 46: menu.DataBind();

Line 47:

Line: 45Estou usando ele numa master page.