Open template manager. Select your template. And try to manage folder.
You can manage folder and able to select a folder from tree.
Folder tree not selectable you are not able to manager folder.
Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/20980
closed as having Pull Request #20981
Set to "closed" on behalf of @franz-wohlkoenig by The JTracker Application at issues.joomla.org/joomla-cms/20980