Bind Treeview Control Problem

<form id="form1" runat="server">
    <div>
        <asp:TreeView runat="server" ID="tvw">
        </asp:TreeView>
    </div>
    </form>

public partial class TestPage : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
    DataTable result = GetData();
    var dt = result.AsDataView();
    dt.Sort = "Level";
          
    //tvw.Nodes.Add(new TreeNode("Root", "-1"));

    TreeNode node = null;
    int previousLevel = 0;
    TreeNodeCollection coll = null;// = tvw.Nodes[0].ChildNodes;

    foreach (DataRowView row in dt)
    {
        int level = int.Parse(row["Level"].ToString());

        if (level > previousLevel)
        {
            previousLevel = level;
            coll = node.ChildNodes;
        }
        node = AddNodes(coll, level, row);
    }
}

private DataTable GetData()
{
    var dt = new DataTable();
    dt.Columns.Add("UserId");
    dt.Columns.Add("Email");
    dt.Columns.Add("FullName");
    dt.Columns.Add("Level");

    var row = dt.NewRow();
    row["UserId"] = "nafees.ahmed";
    row["Email"] = "nafees.ahmed@ibtechnology.com";
    row["FullName"] = "NAFEES AHMED";
    row["Level"] = 0;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "sameerkhanna";
    row["Email"] = "sameerkhanna@ibtechnology.com";
    row["FullName"] = "SAMEER KHANNA";
    row["Level"] = 1;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "juned.mohd";
    row["Email"] = "juned.mohd@ibtechnology.com";
    row["FullName"] = "Mohd Juned	sameerkhanna";
    row["Level"] = 2;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "nafees.ahmed";
    row["Email"] = "nafees.ahmed@ibtechnology.com";
    row["FullName"] = "NAFEES AHMED";
    row["Level"] = 0;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "rana.samir";
    row["Email"] = "rana.samir@ibtechnology.com";
    row["FullName"] = "Samir Rana	sameerkhanna";
    row["Level"] = 2;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "harkesh.deshwal";
    row["Email"] = "harkesh.deshwal@ibtechnology.com";
    row["FullName"] = "Harkesh Deshwal sameerkhanna";
    row["Level"] = 2;
    dt.Rows.Add(row);

    row = dt.NewRow();
    row["UserId"] = "satish.k5";
    row["Email"] = "satish.k5@ibtechnology.com";
    row["FullName"] = "Satish Kumar	sameerkhanna";
    row["Level"] = 2;
    dt.Rows.Add(row);

    return dt;
}

private TreeNode AddNodes(TreeNodeCollection nodes, int level, DataRowView r)
{
    var item = new TreeNode()
    {
        Text = string.Concat("(", r["Level"].ToString(), ") ", r["UserId"].ToString(), r["Email"].ToString(), r["FullName"].ToString()),
        Value = r["Level"].ToString()
    };
    if (null == nodes)
    {
        tvw.Nodes.Add(item);
                
    }
    else
    {
        nodes.Add(item);
    }
    return item;
}
}