The latest "released" version 2.1.1 is not yet compatible to MediaWiki 1.39. One needs either to use REL1_39 or master branch. Using master may be dangerous with future updates.
Unfortunately the extension description does not state an official "Compatibility policy".
Could the maintainer of this extension maybe update the "Compatibility policy" and/or release a version 3.0.0 that is compatible to MediaWiki 1.39?
Thanks in advance!