@mimmin by doing define('WP_DEBUG', false);
you will just hide the error on front end and back-end but in error log file it will continue. So better to solve the issue rather then hiding it.
And solution is very simple… downgrade the PHP version. And this kind of error is mostly showing in PHP 7.3 version. So my advice downgrade to 7.0 or 7.2 till get solved.
And this to be solve by plugins and theme’s developers. ?? Cheers!