Open a comm to the frontend if one isn't already open.

def open(self):
        """Open a comm to the frontend if one isn't already open."""
        if self.comm is None:
            state, buffer_paths, buffers = _remove_buffers(self.get_state())

            args = dict(target_name='jupyter.widget',
                        data={'state': state, 'buffer_paths': buffer_paths},
                        buffers=buffers,
                        metadata={'version': __protocol_version__}
                        )
            if self._model_id is not None:
                args['comm_id'] = self._model_id

            self.comm = Comm(**args)