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Every time it runs, it will stop with this information. I wonder what should I do?
This seems to happen when gaze stabilization is enabled, while fixation checking is not. The solution would be to explicitly get gaze position at least once regardless of the settings. If you take a look at the part of the code where the error comes from (ItemArray.show()), I'm sure you can figure out how to! (And that's part of the exercise, isn't it?)
There's much bigger issues in the world, I know. But I first have to take care of the world I know.