Message of the day:
SS4.3 is released! - https://www.silverstripe.org/blog/announcing-the-release-of-silverstripe-4-3
SilverStripe 4 related information and questions.
Oh, TreeDropdownField only expects a Hierarchy I believe... but that's difficult because it's not an interface (it's an Extension; capital E). As to what it actually expects (read: how coupled that is to
SiteTree) I dunno. But it works with more than just SiteTree (or at least used to) - in that it goes for
Folder as well.