Error in grasp controller.

Hello,
I am trying rqt plugins in simulation.
While trying to save the grasp in grasp controller rqt plugin following error occurs.

```
sr_visualization/sr_gui_grasp_controller/src/sr_gui_grasp_controller/grasp_controller.py", line 258, in accept
    self._target_mutex.acquire()
AttributeError: 'GraspSaver' object has no attribute '_target_mutex'
```