ReviewRequestFieldSet has a global list for storing any registered

fields. If a subclass didn't override that list, then it would share the

list of fields with any other subclass that didn't override it, leading

to problems.
We now use the pattern that the My Account page uses, where we default

the list to None, and then we specifically set it to an empty list on

the subclass during fieldset registration, if it's unset.

Registered into one subclass, and didn't see it in the other.