Ensure that the QueryRoot block is valid.

def validate(self):
        """Ensure that the QueryRoot block is valid."""
        if not (isinstance(self.start_class, set) and
                all(isinstance(x, six.string_types) for x in self.start_class)):
            raise TypeError(u'Expected set of string start_class, got: {} {}'.format(
                type(self.start_class).__name__, self.start_class))

        for cls in self.start_class:
            validate_safe_string(cls)