Newer Flexmail accounts cause error in webform handler

Problem/Motivation

If you have a newer Flexmail account and try to attach the "Flexmail handler" to a webform, you'll get an error in the console:
"403: Access to this resource is denied (You are either not permitted to access this resource or it is not included in your current plan. Read more about Flexmail's plans on our website)"

This is caused by the fact that API-endpoints "/preferences" and "/interest-labels" are deprecated and no longer available in newer accounts.

Proposed resolution

Make the preferences / interest label parts optional.