code: def get_unit_by_index(self, unit_index) -> typing.Optional['BaseUnit']:
          
          if unit_index in self._section_group['units'].keys():
              if unit_index not in self.__units.keys():
                  _category = self.units_class_enum[self.group_category]  # type: ignore
                  self.__units[unit_index] = _category(self.d, self.l10n, self.coa_color,
                                                       self.country_index,
                                                       self.group_category,
                                                       self.group_index, unit_index)
              return self.__units[unit_index]
          return None
docs: Args:
              unit_index: index of unit
  
          Returns: a unit of this group