I'm placing the treeview inside "asp:WizardStep" and I keep getting "Object reference not set to an instance of an object.".
In one post I found trick to add the tree in panel and add the tree and the events to the panel in "Page_Init".
<asp:Wizard ID="Wizard1" runat="server">
<asp:WizardStep runat="server" Title="Step 1">
<asp:Panel ID="Panel1" runat="server">
This trick work only if the ExpandMode of the nodes is "ServerSide" but not if the ExpandMode is "ServerSideCallBack"
You can read an old discussion at