Reproduceable on testwiki: enabling both beta features and trying to edit page source results in Uncaught TypeError: Cannot read property 'urlProtocols' of null. That corresponds to urlProtocols = new RegExp( mwConfig.urlProtocols, 'i' ), in resources/mode/mediawiki/mediawiki.js. Attempts to quickly fix the issue with https://gerrit.wikimedia.org/r/#/c/370950/ and https://gerrit.wikimedia.org/r/#/c/370954/ didn't work. There is clearly some module dependency issue, but we were not able to identify it during our deployment window.
Is it possible to detach the regular editor and the new wikitext editor versions? It's really frustrating to leave folks waiting on the regular editor version, because that's what folks were asking for in the proposal. I want to keep working on the new editor version too, but I'd like to get people using this.
Found another teeny bug while testing this - NWE shows the 'Syntax Highlighting' tab in the dropdown and lets you enable it even if the user has not explicitly enabled the Beta feature. I'm not sure if this is intentional.