Hi,
That code is found here.
https://themes.trac.www.ads-software.com/browser/hueman/3.2.12/option-tree/includes/ot-meta-box-api.php#L353
It’s not undefined and should not be causing fatal error.
1) You can use FTP to delete Hueman theme from wp-contents/themes/
2) Re-install latest theme and addon plugin from www.ads-software.com
If you are still having fatal error, check for plugin conflict, by disabling your other third partly plugins.
If you are using a child theme with customization, you should be updating your customization with reference from Hueman Theme.
Thank you