Added a lock to start_preview and caught errors

    def start_preview(self, fullscreen=True, window=None) -> bool:

        """Start the on board GPU camera preview."""

        logging.info("Starting the GPU preview")

        self.start_stream_recording()

        try:

            with self.lock:

                self.camera.start_preview(fullscreen=fullscreen, window=window)

                self.state['preview_active'] = True

                time.sleep(0.2)

        except picamera.exc.PiCameraMMALError as e:

            logging.error("Suppressed a MMALError in start_preview. Exception: {}".format(e))

        except picamera.exc.PiCameraValueError as e:

            logging.error("Suppressed a ValueError exception in start_preview. Exception: {}".format(e))

        return True

    def stop_preview(self) -> bool:

                    format='mjpeg',

                    quality=self.config['jpeg_quality'],

                    bitrate=-1, # RWB: disable bitrate control 

                    # (bitrate control makes JPEG size less good as a focus

                    # metric)

                    splitter_port=splitter_port)

            except picamera.exc.PiCameraAlreadyRecording:

                logging.info("Error while starting preview: Recording already running.")