Add parent menu slug to add_submenu_page to avoid PHP 8.2 deprecation notices

[claytoncollie]

Description of the Change

This change fixes PHP 8.2 deprecation notices

Deprecated: strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated in /Users/claytoncollie/www/10upexperience/app/public/wp-includes/functions.php on line 7053

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /Users/claytoncollie/www/10upexperience/app/public/wp-includes/functions.php on line 2165

Deprecated: strip_tags(): Passing null to parameter #1 ($string) of type string is deprecated in /Users/claytoncollie/www/10upexperience/app/public/wp-admin/admin-header.php on line 36

How to test the Change

1. Create a fresh install of WordPress
2. Clone repository and checkout branch
3. Set WP_DEBUG and WP_DEBUG_DISPLAY to true
4. Visit the wp-admin to look for the errors
5. Make sure you can visit the about 10up screen --> http://localhost:10003/wp-admin/admin.php?page=10up-about

Changelog Entry

Fixed - Passing null as parameter 1. Define the parent menu slug when registering the submenu screen

Credits

Props @claytoncollie

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[pabamato]

I was about to report the same issue.
@darylldoyle this passed my tests (PHP 8.2 - WP 6.3.1)