Returns True or False indicating whether or not the specified
        target is present and valid.

        `target_id` is a target ID (or None for the first target)

def target_is_valid(self, target_id=0):
        """
        Returns True or False indicating whether or not the specified
        target is present and valid.

        `target_id` is a target ID (or None for the first target)
        """
        try:
            target = self._target(target_id=target_id)
        except:
            return False
        return target['state'] != "invalid"