Solution:
This issue appears to have been first introduced in version 4.7, and is due to wp-includes/class-wp-list-util.php not checking if an object property is set.
Anyway… To remove this warning, edit your wp-config.php and change this:
define( 'WP_DEBUG', true );
…to this (or comment out the WP_DEBUG line altogether ):
define( 'WP_DEBUG', false );
Alternatively, if you want to leave WP_DEBUG enabled but limit its output to wp-content/debug.log, you can add this instead:
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );
- WP_DEBUG (and especially WP_DEBUG_LOG should not be enabled in a production environment, or on any publicly-accessible site (such as QA/staging) unless you know what you’re doing (else sensitive information may be revealed).