Hi, just because it took me some time to figure it out, maybe it saves somebody else some time.
For whatever reason, in a recent update of Things, the JXA function
lists() (is this called a getter?) of the Things app is now a property. I show what I mean with some code.
things = Application("Things3") // this does not work anymore upcoming = things.lists() // instead you have to write upcoming = things.lists // for the other things, () is still working projects = things.projects() tags = things.tags() areas = things.areas()