Cannot create custom fields

Hi folks,
I have the latest version of s2member and the pro extention installed but when I click on the “create registration field” button on the custom fields popup I get the following javascript error:

Unable to get property 'stringify' of undefined or null reference
which seems to relate to this part of the code:
$fields.val(((fields.length>0)?$.JSON.stringify(fields):""))}

I’ve tried editing the .htaccess file as described here
https://github.com/WebSharks/s2Member/issues/144#issuecomment-43431997
But no joy

Any thoughts?

https://ww.wp.xz.cn/plugins/s2member/