Args:
            unit_index: index of unit

        Returns: a unit of this group

def get_unit_by_index(self, unit_index) -> typing.Optional['BaseUnit']:
        """
        Args:
            unit_index: index of unit

        Returns: a unit of this group
        """
        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