Preview throws fatal error when children present

Also seems to occur in 3.0.x

Here's a patch. I will fire up a MR in a minute.

However, it looks like this path only treats the symptom. The real issue is that in a preview context, some of the expected variables seem to be missing.

Oddly, I can no longer reproduce this on Drupal 11.1.3

Was able to replicate on core 11.1.6

Followed the steps and this issue is also present in core 11.1.8.
Applied the patch MR!82.
Result :
1. Added New Book preview is visible now (before applying patch the error was displayed for the preview page)
2. Added single as well as multiple child pages . Preview is working
3. Verified the previews of child pages - Working
4. Verified Adding child for child page (nested) - Previews working
No console errors
RTBC ++
Keeping in Need Review for further code review

Let’s add a simple test case somewhere

up-ported to 3.0.x branch too.