nodes not added to TreeNodeCollection of tree view

I have a tree view the will be populated by folder & files by TreeNodePopulate function. When I run the code tree view will be populated correctly but when I want to filter tree view by function (FindNodeByValue) I have problem, it seems TreeNodeCollection just has 1 node that I had made handy in the first of code (Page_Load ) and other nodes that will be produced by TreeNodePopulate function  not added in TreeNodeCollection, please help how I can improve code & solve it.

The code is like below:

<pre>protected void Page_Load(object sender, EventArgs e)
{
    TreeView1.Nodes[0].Value = Server.MapPath("Document/Detail Engineering");


}


protected void TreeView1_TreeNodePopulate(object sender, TreeNodeEventArgs e)
{
    if (IsCallback == true)
    {
        if (e.Node.ChildNodes.Count == 0)
        {
            LoadChildNode(e.Node);
        }
    }
}

private void LoadChildNode(TreeNode node)
{
    DirectoryInfo directory = null;
    directory = new DirectoryInfo(node.Value);

    foreach (DirectoryInfo subtree in directory.GetDirectories())
    {
        TreeNode subNode = new TreeNode(subtree.Name);
        subNode.Value = subtree.FullName;
        try
        {
            if (subtree.GetDirectories().Length > 0 | subtree.GetFiles().Length > 0)
            {
                subNode.SelectAction = TreeNodeSelectAction.SelectExpand;
                subNode.PopulateOnDemand = true;
                subNode.NavigateUrl = "#";
            }
        }
        catch (Exception ex)
        {

        }
        node.ChildNodes.Add(subNode);
    }
    foreach (FileInfo fi in directory.GetFiles())
    {
        TreeNode subNode = new TreeNode(fi.Name);
        node.ChildNodes.Add(subNode);
        subNode.NavigateUrl = "media/" + fi.Name.ToString();
        //subNode.NavigateUrl = Server.MapPath(fi.Name.ToString());
    }
}



private IEnumerable<TreeNode> FindNodeByValue(TreeNodeCollection nodes, string searchstring)
{

    foreach (TreeNode node in nodes)
    {
        if (node.Value.IndexOf(searchstring,
              StringComparison.CurrentCultureIgnoreCase) >= 0)
            yield return node;
        else
        {
            foreach (var subNode in FindNodeByValue(node.ChildNodes, searchstring))
                yield return subNode;
        }
    }
    //TreeView1.Nodes.Clear();
}
protected void Button1_Click(object sender, EventArgs e)
{

    var query = FindNodeByValue(TreeView1.Nodes, fieldFilterTxtBx.Text);
    if (query != null)
    {
        //TreeView1.Nodes[0].Expand();
        //TreeView1.Nodes.Clear();
        foreach (TreeNode node in query.ToList())
        {
            TreeView2.Nodes.Add(node);

        }



        TreeView2.Visible = true;
        TreeView2.ExpandAll();
    }

    else
    {

        Label1.Text = "No file found";

    }
}