I get this error whenever my camera is not being detected.
Here's the stack trace in the GRIP UI.
Make sure that your camera is being detected -- you can view the image GRIP is reading from your camera by clicking the eye next to the Image object. If no image is shown, that means your camera is not being read properly. Check if any other programs (Skype or any video chat program, or mjpeg-streamer if headless) recognizes the webcam. You can also try switching USB ports and then restarting GRIP.