When attempting to insert citations in source, everything works properly until the citation has been formatted and I click "Insert", at which point the window closes itself without inserting any code. The tool (is that the right term?) works properly when using the visual editor. I've reproduced this error on two different computers running Mac OS 10.14.6, using either Safari or Firefox.
Other editors appear to have also reproduced the bug, as attested in this Teahouse thread.