Undefined property notice when saving Custom CSS when this plugin is active in Pressbooks

[SteelWagstaff]

Description
On the Pressbooks network with WP_DEBUG set to true, we see an Undefined property: stdClass::$edit_page in /srv/www/integrations.pressbooks.network/releases/20210427183011/web/wp-includes/capabilities.php on line 172 notice whenever we save Custom CSS if this plugin is active.

Steps to Reproduce

1. Open a book on a Pressbooks network with WP_DEBUG set to true and the Candela Citations plugin active and click Appearance -> Custom Styles
2. Save a change to the CSS
3. Observe a white screen briefly displaying the notice indicated above before you're returned to the Pressbooks interface

candela_permission_error.mp4

Expected behavior:
No notice is displayed

Actual behavior:
A notice is diplayed.

Notes
Here's the line cited by the notice: https://github.com/WordPress/WordPress/blob/267061c9595fedd321582d14c21ec9e7da2dcf62/wp-includes/capabilities.php#L172

Like #7, this issue is only present when Candela Citations is activated, and disappears when that plugin is deactivated.

More details at pressbooks/pressbooks#2149

[SteelWagstaff]

I don't know how this plugin works very well yet, but did notice that the plugin contains a 'save' function which attempts to check whether current_user_can permissions allow them to edit_page for a specific post_id:

candela-citation/inc/class-citation.php

Lines 430 to 432 in 083e08a

	if ( ! current_user_can( 'edit_page', $post_id ) ) {
	return $post_id;
	}

.