Object reference not set to an instance of an object.

Hello,

i am using the infragistics web tree controled and on my page load I am having  this error
Object reference not set to an instance of an object. the line that is causing it is below

igTree.Nodes[0].Selected =true ;

here is my page_load code
 protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            lblId.Text = Request["ParentTitle"];
                               
             LoadTree(Request["ParentTitle"]);
            igTree.Nodes[0].Selected =true ;
           
            PopulateUnassignedData();
                                   
         }
         AdminID = Session["admin_id"].ToString();
     }