4e84027dbc2e updated the grouping fields method to receive the grouped
instance but it did not adapt to the returned type which is a list.
Closes#12718
(grafted from 099eaaf2255b752db61c8c91441e430bf148795a)
--HG--
branch : 7.0
f6e6e1921a54 changed the property into a method with the grouped instance as
parameter. So other modules extending the method must be updated.
Closes#12634