Editing the top book page in a book corrupts the book

Problem/Motivation

Steps to reproduce

Install drupal 11 with the Book module and Edit +
Create a book page, and under Book choose Create a new book
Create a child book page, and save it.
Enter edit mode
Edit the child book page content - no problem
Go to the parent book page (main page)
Enter edit mode
Edit the parent book page content
Click outside of the editing area (no need to save changes)

Drupal produces an error "Oops, something went wrong. Check your browser's developer console for more details."
Browser's developer console lists the following error: Drupal.AjaxError {message: '\nAn AJAX HTTP error occurred.\nHTTP Result Code: 20…al_ajax&ajax_form=1\nStatusText: OK\nResponseText: ', name: 'AjaxError', stack: 'Error\n at http://d11.gargtech.com/sites/volo.ne…LaDADvNkEnCtYRLG4RrHxWTENQt4xd5_AVxb66I:241:19695'}

At this point, if you click Discard Changes the damage can be undone, but if you do NOT and simply click away, the parent page is unusable (can't load) and must be deleted to restore functionality. To be clear, this damage is done WITHOUT PRESSING SAVE, which seems very odd!

When the main page fails to load, it produces an error log entry: "TypeError: Drupal\book\ProxyClass\BookManager::bookSubtreeData(): Argument #1 ($link) must be of type array, null given, called in /volo/drupal11/web/modules/contrib/book/src/BookOutline.php on line 49 in Drupal\book\ProxyClass\BookManager->bookSubtreeData() (line 209 of /volo/drupal11/web/modules/contrib/book/src/ProxyClass/BookManager.php)" along with a few less concerning warnings.

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes