As a rule, callers of ResourceLoader::getModule() in includes/resourceloader/ResourceLoader.php do not guard against a null return value, and simply assume it is a ResourceLoaderModule object.
Exceptions on attempts to call methods on non-objects are the current (superficial) cause of missing CSS / JS on http://en.hhvm.beta.wmflabs.org/wiki/Main_Page. The deeper cause is probably that en.hhvm.beta.wmflabs.org and en.wikipedia.beta.wmflabs.org share the same DB but load a different set of extensions. Regardless, if it is possible for ResourceLoader::getModule to return null, callers should check for it and react appropriately.
Version: 1.23.0
Severity: normal