Most feather messages are benign--it's especially poor at interpreting functions which support variable arguments/data types, or variables which implicitly cast data types intentionally.
I can confirm that Xtend is working in your version of GameMaker Studio. Most likely, something about the project configuration isn't automatically being set up the way it should. Line 141 of xtend assumes there is at least one room in the project and it has a viewport of the index assigned in xtend_config. Both of these should always be true out of the box, but if not, you can always assign your first room and enable the viewport manually.