Fix unset variable

[Rahmon]

Description

In the gutenberg_meta_box_post_form_hidden_fields function the active_post_lock variable never was defined. This function was added in #2804 by @BE-Webdesign.

Closes #4714

How Has This Been Tested?

This has been tested with "npm test", "npm run test-e2e" and manually on Chrome and Firefox.

Types of changes

Bug fix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows has proper inline documentation.

[aduth]

Can you explain a bit more the context for the specific change to set a post lock if one does not exist? I am not sure whether we want to be incurring side-effects as part of this code, vs. simply dropping the dead code as was proposed in #4714.

[aduth]

As a productivity quality-of-life tip: You can flag your pull request to automatically close related issues with specific keywords, e.g. "Closes #4714"

See also: https://help.github.com/articles/closing-issues-using-keywords/

[youknowriad]

This field is probably something related to Post Locking and for the meta boxes work @BE-Webdesign was trying to be as close as possible to the classic's editor markup to avoid issues with meta boxes trying to access dom nodes etc... I don't see this field used by meta boxes in general. Fine to remove for me.

Post locking should be worked on separately anyway.