There is two ways to hide this warning
1. Go to your host cpanel and in php-settings turn display_errors OFF
2. Open with ftp your wp-config.php and
replace line:
define(‘WP_DEBUG’, false);
with this:
ini_set(‘display_errors’,’Off’);
ini_set(‘error_reporting’, E_ALL );
define(‘WP_DEBUG’, false);
define(‘WP_DEBUG_DISPLAY’, false);