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My experiment quits itself - ValueError

Hello all

First, I’ll start by explaining my experiment: participants get a black screen with white dots moving around. When a dot touches the edge of the screen, it bounces back. On half of the trials, one of the dots makes an abrupt direction change at a random moment in the trial. Participants are then asked if they saw that. This all is working. So no problems here (luckily)!

But…

When I run it, everything seems to be fine at first. But then, at a certain moment (sometimes after ten trials, sometimes after two blocks, always a different moment), it just stops running and quits itself. It then gives the error as you can see in the picture.

I have no idea where it comes from because it is always at a different moment in time.

Thanks in advance!

Kind regards,
Marie