def do_execute(self):
        """
        The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str
        """
        directory = str(self.resolve_option("dir"))
        if not os.path.exists(directory):
            return "Directory '" + directory + "' does not exist!"
        if not os.path.isdir(directory):
            return "Location '" + directory + "' is not a directory!"
        collected = []
        result = self._list(directory, collected)
        if result is None:
            for c in collected:
                self._output.append(Token(c))
        return result
The actual execution of the actor.

        :return: None if successful, otherwise error message
        :rtype: str