This method is supposed to accept null to allow a copy of the formatter to be returned reset to the original state of having no resolver fields. The docs say:
"@param resolverFields the new set of resolver fields, null if no fields"
which was written to indicate that resetting to null is permitted.
The fix is to check for null and return a copy of the formatter. Note that there are two variations of the method which need fixing.
This should be backported to 8u.