Returns a ``Sii`` instance from ``self.container`` if it is valid,
        if all elements of ``self.container[specfile][identifier] are
        ``Sii.isValid == False`` then ``None`` is returned.

        :param specfile: a ms-run file name
        :param identifier: item identifier ``Sii.id``

        :returns: ``Sii`` or ``None``

def getValidItem(self, specfile, identifier):
        """Returns a ``Sii`` instance from ``self.container`` if it is valid,
        if all elements of ``self.container[specfile][identifier] are
        ``Sii.isValid == False`` then ``None`` is returned.

        :param specfile: a ms-run file name
        :param identifier: item identifier ``Sii.id``

        :returns: ``Sii`` or ``None``
        """
        for item in self.container[specfile][identifier]:
            if item.isValid:
                return item
        else:
            return None