Unknown Error - trying to change permissions to page after removing it's creator #2687

Describe the bug
After removing a user and trying to change permissions for pages he created - you will end with "An Error Occurred" and you can't view this page anymore, same for books and probably shelves. It seems that changing permissions triggers this apocalypse. Migrating ownership didn't change anything

Steps To Reproduce
Steps to reproduce the behavior:

1. Create new account, e.g $[test]
2. Log in using account $[test]
3. Using $[test] account create new $[page]
4. Log in as admin and delete $[test] account
5. Change permissions for this $[page]
6. Tadam, now you can't access $[page] anymore and you see only:
   "An Error Occurred
   An unknown error occurred"

Same with books. Reproduce steps above with $[book] instead of $[page] and:

• you will be able to read pages in that book from e.g homepage
• you won't be able to open $[book]

Expected behavior
You should be able to migrate ownership after removing user

Your Configuration (please complete the following information):

• Exact BookStack Version (Found in settings): 21.04
• PHP Version: 7.3.27
• Hosting Method (Nginx/Apache/Docker): Docker, using linuxserver image of bookstack and mariadb