Description
A missing variable causes a PHP warning during WordPress Core updates.

The error message is as follows:

PHP Warning: compact(): Undefined variable $new_version in [...]/wp-content/plugins/mainwp-child/class/class-mainwp-child-actions.php on line 655

The update itself is not interrupted and the notification email about the automatic update gets delivered normally.

Steps To Reproduce

• To be safe use a fresh install of WordPress
• Install MainWP Child plugin and activate
• Connect MainWP Child to a MainWP Dashboard (it won't work without this step)
• Install and activate the plugins "Core Rollback" and "WP Console"
• Use "Tools" > "Rollback Core" to select WordPress version 6.8.1 and click on "Rollback", when redirected press the button "Re-install version 6.8.1".
• To immediately force trigger the auto update process go to "Tools" > "WP Console" and run the following command:
  wp_maybe_auto_update(); You should immediately be shown the PHP warning message.
• As an alternative you can also wait for wp cron to trigger the update process (takes up to 12 hours), but then you need

  define( 'WP_DEBUG', true );
  define( 'WP_DEBUG_LOG', true );
  define( 'WP_DEBUG_DISPLAY', false );
  

  set in your wp-config.php file. After the update /wp-content/debug.log for the PHP warning message.

Expected behavior
Of course the expected behavior is the update going through without any PHP error messages or warnings.