Page Forms includes the Dynatree JS library for use by the "tree" input type. The Dynatree code is from 2013; this library is now defunct, and has been replaced by the Fancytree JS library:
http://wwwendt.de/tech/fancytree/
It would be good if Page Forms could switch to using the Fancytree library. I think the two libraries work more or less the same, so hopefully switching to it will not be too hard.
This is part of the larger project to upgrade all JS libraries in Page Forms, T184251.