System.Guid issue in c#

Unable to cast object of type 'System.Guid' to type 'System.String'

DataTable dtMenu = new DataTable();
                   DataSet ds = new DataSet();


                   XmlDataSource xmlDataSource = new XmlDataSource();
                   xmlDataSource.ID = "XmlSource1";
                   xmlDataSource.EnableCaching = false;

                   dtMenu = GetMenuTable();
                   ds.Tables.Add(dtMenu);

                   ds.DataSetName = "Menus";
                   ds.Tables[0].TableName = "Menu";

                   DataRelation relation = new DataRelation("Parentchild", ds.Tables["Menu"].Columns["ChildId"], ds.Tables["Menu"].Columns["ParentId"], true);

                   relation.Nested = true;
                   ds.Relations.Add(relation);

                   xmlDataSource.Data = ds.GetXml();


                   xmlDataSource.TransformFile = Server.MapPath("~/Menu.xslt");


                   xmlDataSource.XPath = "MenuItems/MenuItem";


                   NavigationMenu.DataSource = xmlDataSource;
                   NavigationMenu.DataBind();

                   linkuaername.Text =(string)Session["username"];    // here i got an error 
                   Authenticatemenu(linkuaername.Text);