Ref T13625. See that task for discussion.
Currently, the Viewer when performing revision updates in response to commits may be an arbitrary low-privilege user (an Application, a disabled User, a bot, a mailing list, etc).
Today, this leads to an exception when trying to make API calls.
Ideally, we probably would not perform the update in these cases. However, performing the update isn't a policy violation and is generally less surprising than not performing it, so continue performing it for now: just use the omnipotent user to interact with the API.