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()[3]
// instead you have to write
upcoming = things.lists[3]
// for the other things, () is still working
projects = things.projects()
tags = things.tags()
areas = things.areas()