Fix crash in IsFamilyFilterMatchEvent filter rule

The prepare() method built the matching event handle set in
self.selected_handles but then tried to update self.events, an
attribute that is never defined. This raised an AttributeError
whenever the "Events of families matching a <family filter>" rule
was applied, breaking the filter entirely.

Reported at:
https://gramps.discourse.group/t/crash-of-an-event-filter-using-a-functional-family-filter/9733

Author: dsblank

FilterRules/isfamilyfiltermatchevent.py

@@ -92,7 +92,9 @@ def prepare(self, db: Database, user):
         if self.MFF:
             for family in db.iter_families():
                 if self.MFF.apply_to_one(db, family):
-                    self.events.update([e.ref for e in family.get_event_ref_list()])
+                    self.selected_handles.update(
+                        [e.ref for e in family.get_event_ref_list()]
+                    )
 
     def apply_to_one(self, db: Database, event: Event) -> bool:
         """