In my DotNetNuke 6 custom module the treeview crashed on postback in view mode, but worked properly in edit mode
In my case I had an UpdatePanel in the ascx control. To update all my modules to work properly in DotNetNuke 6 I had to get rid of most of the Update Panels. This may not be necessary in your case.
Whenever you have a module that posts back you have to set the cache to 0 in the module settings.