We don't think this ever appeared in production.
If you enable Flow for a namespace and visit a new page in it, you get
Catchable fatal error: Argument 1 passed to Flow\Templating::renderHeader() must be an instance of Flow\Model\Header, null given, called in /home/spage/projects/core/extensions/Flow/includes/Block/Header.php on line 177 and defined in /home/spage/projects/core/extensions/Flow/includes/Templating.php on line 208
Version: master
Severity: normal