Hi David,
I set this up on Ubuntu 18.10 (no 18.04 left on my machines, sorry) and although I do not get error messages from Document Viewer, emacs is not invoked. So, not working for me, but in a different way.
Each to his own, and there are pros and cons each way, but I eventually became tired of all the difficulties with Ubuntu and switched to Debian where everything is good. It may not be helpful to you to hear that, but it did solve all my lilypond issues of various sorts. However, there are many committed Ubuntu users on the list.
Andrew