Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'behavior') and self.behavior is not None:
            _dict['behavior'] = self.behavior
        if hasattr(self, 'dialog_node') and self.dialog_node is not None:
            _dict['dialog_node'] = self.dialog_node
        if hasattr(self, 'selector') and self.selector is not None:
            _dict['selector'] = self.selector
        return _dict