window.mw.config.get( 'LexemeLanguageCodePropertyId' ); is read in tests/selenium/specs/new.js via JS - occasional flakyness of this test suggests that sometimes this happens too quickly, before the ResourceLoader(?) set the value accordingly.
Error message is a bit misleading
10:02:00 1) NewLexeme:Page with default LexemeLanguageCodePropertyId configured "before all" hook for "request with "createpage" right shows form": 10:02:00 unknown error: Cannot read property 'get' of null 10:02:00 running chrome 10:02:00 Error: An unknown server-side error occurred while processing the command. 10:02:00 at Promise.F (node_modules/core-js/library/modules/_export.js:36:28) 10:02:00 at execute(<Function>) - at Context.<anonymous> (extensions/WikibaseLexeme/tests/selenium/specs/new.js:30:43)
e.g. https://integration.wikimedia.org/ci/job/mwext-mw-selenium-node-composer-jessie/1380/console