New documents created via the wiki editor cause a non-breaking error to print to the console when autosaving

After creating a new document, before saving to an official version, the autosave function attempts to check whether the existing version must be merged with the current state before autosaving. In such cases where there is no such previously saved versions, this results in a 404 from the REST API.

This error is not handled properly, which results in a reference to an undefined variable which complains on the console, but does not otherwise cause the page to stop functioning.

Saving via the 'save and continue' button alleviates the problem.