> var_dump($wgUsersNotifiedOnAllChanges); array(1) { [0]=> string(0) "" }
was giving
TypeError: Argument 1 passed to EmailNotification::compose() must implement interface MediaWiki\Mail\UserEmailContact, bool given, called in /srv/mediawiki/w/includes/mail/EmailNotification.php on line 297
https://github.com/miraheze/mediawiki/pull/7044 is my attempt at stopping it.