Invoke an application callback.

def trigger_callback(self, sid, namespace, id, data):
        """Invoke an application callback."""
        callback = None
        try:
            callback = self.callbacks[sid][namespace][id]
        except KeyError:
            # if we get an unknown callback we just ignore it
            self._get_logger().warning('Unknown callback received, ignoring.')
        else:
            del self.callbacks[sid][namespace][id]
        if callback is not None:
            callback(*data)