To set the visibility of the ListRootContainer element, you will need to edit the control template of the RadTreeViewItem control. I've prepared a small sample project to demonstrate how this can be done in the Office_Black theme when using the implicit styles theming approach with the NoXaml binaries. Do note, however, that if you're using another theme(s) you need to extract and modify the respective template(s) accordingly.
Please have a look and let me know if this delivers the desired result.
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