TypeError: Cannot set properties of undefined (setting 'size')

URL of experiment: Pavlovia

Description of the problem: I created an experiment that starts with a Pavlovia survey (json file) and then shall present participants with the first task (a slider component from PsychoPy). However, when I finish the survey and click ‘complete’, my browser switches from fullscreen to windowed mode, Pavlovia flashes an error message ('TypeError: Cannot set properties of undefined (setting ‘size’)… and then I’m redirected to an Error 404 nginx page.

I’ve already looked for weird-looking lines within the code, and tried to delete the slider component. But so far, the error persists. This is my first PsychoPy project and my coding knowledge is pretty basic. So any helpful hints would be very much appreciated. Thanks!