Pressing Ctrl-F5 is a good start to clear it from cache. It should be compatible, I used it in during a demo recently.
However a lot can go wrong during configuration. if you believe you're configuration is fine you should file a support ticket.
The Treeview widget is not compatible with Mendix 6 so you can keep pressing Ctrl-F5.
Could not create widget TreeView.widget.TreeView TypeError: x.entitystyle is undefined
The sample project still contains formloaders that are also pretty deprecated.
This widget is a heavy native dojo widget which is powerful but pretty complex, I would prefer a fresh implementation with just an entity, association and an attribute, should be enough.