          if isinstance(self.field, models.NullBooleanField):
              yield {
                  'selected': self.lookup_val2 == 'True',
                  'query_string': cl.get_query_string({
                          self.lookup_kwarg2: 'True',
                  }, [self.lookup_kwarg]),
                  'display': _('Unknown'),
              }
  
  FieldListFilter.register(lambda f: isinstance(f,
 
</s> remove                         self.lookup_kwarg: lookup,
 </s> add                     self.lookup_kwarg: lookup, </s> remove                                     self.lookup_kwarg: lookup}),
 </s> add                     self.lookup_kwarg: lookup}), </s> remove                                     param_dict, [self.field_generic]),
 </s> add                     param_dict, [self.field_generic]), </s> remove         bool(f.rel) if hasattr(f, 'rel') else
        isinstance(f, models.related.RelatedObject)), RelatedFieldListFilter)
 </s> add     bool(f.rel) if hasattr(f, 'rel') else
    isinstance(f, models.related.RelatedObject)), RelatedFieldListFilter) </s> remove         }, messages.SUCCESS)
 </s> add             }, messages.SUCCESS)
                    self.lookup_kwarg2: 'True',
          if isinstance(self.field, models.NullBooleanField):
              yield {
                  'selected': self.lookup_val2 == 'True',
                  'query_string': cl.get_query_string({
                    self.lookup_kwarg2: 'True',
                  }, [self.lookup_kwarg]),
                  'display': _('Unknown'),
              }
  
  FieldListFilter.register(lambda f: isinstance(f,
keep keep keep keep replace keep keep keep keep keep
 Fixed #21288 -- Fixed E126 pep8 warnings 
django/contrib/admin/filters.py