Can't offer much help except to say "me too". I've noticed that behaviour as well. When I was setting up a lot of custom fields, I created them and didn't associate types. When I received the error, I would restart WHD and keep going from there. Once all the fields were created, I would then begin associating them to all the request types and restart WHD when this failed too.
I know that is a pain, but since then I haven't actually set up any new fields, so it usually only happens in batches.
Hopefully someone here has a solution.
I'm receiving this error as well when creating a custom field and enter text into the Options section.
HotFix 2 is out on the customer portal. This HotFix addresses the null pointer exception error when creating custom fields.