Hey @Misho and @childrenscancer,
I could not replicate this issue both in multisite and single install and both running PHP 7.2.0. This issue is common in PHP 7.2 but I believe there's something specific within 7.2 setup in some sites that is causing this.
Does this error affect the functionality of the Custom 404 extension? And, for the meantime, can we hide the PHP warnings in the front-end by inserting this code in wp-config.php?
If you already have that, please currently ignore the warning if it's not affecting the functionality of the plugin.
I'll report this in our issue tracker so our development team would be made aware that this issue happens in some sites that is using PHP 7.2.