Reproduction steps

1. Create a new book
2. Create a version
3. Go to the book top page
4. In the "Versions" menu, select "Convert to versioned"

Result

After the redirection, the page is empty and the navigation panel is absent.

When looking at the objects, the BookPageClass object is absent, which is the reason for the weird behaviour.

Explanation

When switching the top page to versioned, the unversioned page is copied as versioned, but the top page is "reseted" with a template, removing all the relevant objects from it (Book/LibraryClass but also the PublicationClass).
Instead:

• the top page should remain mostly the same to keep its objects
• the versioned page should be copied from it but remove Book/LibraryClass and PublicationClass objects
• when switching a versioned page to unversioned, the Book/LibraryClass and PublicationClass objects should be kept the same on the top page.