Notice: Undefined index

Not a huge deal, shouldn’t hurt the function of the code but you can remove that error by doing this:

CHANGE:
if ($_value['id']) register_setting( 'media', $_value['id'] );

To:
if (isset($_value['id']) && $_value['id']) register_setting( 'media', $_value['id'] );

Or easier: upgrade to the latest development version from https://www.ads-software.com/extend/plugins/easy-fancybox/developers/ ??