How to render an ASP.NET TreeView programmatically?

Question

I am trying to dynamically create a basic ASP.NET TreeView control. But when I call RenderControl() to get its output, ASP.NET throws null reference exceptions. What is ASP.NET doing behind the scenes when I declare a TreeView in .ascx/aspx that is NOT happening here?

Broken:

    protected override void DoRender(HtmlTextWriter output)
    {
        TreeView treeview = new TreeView();
        treeview.SkipLinkText = String.Empty;  //omit this for exception on get_SpacerImageUrl
        treeview.ImageSet = TreeViewImageSet.BulletedList;

        TreeNode node = new TreeNode("Node 1");
        node.ImageUrl = string.Empty;
        node.ChildNodes.Add(new TreeNode("Subnode 1"));
        node.ChildNodes.Add(new TreeNode("Subnode 2"));
        treeview.Nodes.Add(node);

        node = new TreeNode("Node 2");
        node.ChildNodes.Add(new TreeNode("Subnode 1"));
        node.ChildNodes.Add(new TreeNode("Subnode 2"));
        node.ChildNodes.Add(new TreeNode("Subnode 3"));
        treeview.Nodes.Add(node);

        treeview.RenderControl(output);
    }

Works fine:

            <asp:TreeView ID="tvMenu" runat="server">
            <Nodes>
                <asp:TreeNode Text="Node 1">
                    <asp:TreeNode Text="Section 1"></asp:TreeNode>
                    <asp:TreeNode Text="Section 2"></asp:TreeNode>
                    <asp:TreeNode Text="Section 3"></asp:TreeNode>
                </asp:TreeNode>
                <asp:TreeNode Text="Node 2">
                    <asp:TreeNode Text="Section One"></asp:TreeNode>
                    <asp:TreeNode Text="Section Two"></asp:TreeNode>
                    <asp:TreeNode Text="Section Three"></asp:TreeNode>
                </asp:TreeNode>
            </Nodes>
        </asp:TreeView>

The exception: [NullReferenceException: Object reference not set to an instance of an object.] System.Web.UI.WebControls.TreeView.GetImageUrl(Int32 index) +3089 System.Web.UI.WebControls.TreeNode.Render(HtmlTextWriter writer, Int32 position, Boolean[] isLast, Boolean enabled) +2329