Set a default self.to_field_name for FilterChoiceField

netbox/utilities/forms.py

@@ -596,7 +596,7 @@ class FilterChoiceField(forms.ModelMultipleChoiceField):
         # Modify the QuerySet of the field before we return it. Limit choices to any data already bound: Options
         # will be populated on-demand via the APISelect widget.
         if bound_field.data:
-            kwargs = {'{}__in'.format(self.to_field_name): bound_field.data}
+            kwargs = {'{}__in'.format(self.to_field_name or 'pk'): bound_field.data}
             self.queryset = self.queryset.filter(**kwargs)
         else:
             self.queryset = self.queryset.none()