RevisionActionPermissions::isAllowed() accepts null to figure out permissions for things that are not yet an existing revision; to figure out if we are actually allowed to create a new post/topic/summary/...
However, part of the permissions check is also looking at the root (topic), and now also board level.
If the topic locked/moderated, we shouldn't allow anything new to be added.
Currently, for new items (null), we can't trace back the root post, so we can't check those permissions.
It's possible to create a summary for a locked topic, for example. But once it's been created, you can't edit it anymore (because then we can figure out that thing's root)