I am having the same error message when piloting a study that has no loops and just text and images. using the developer tools I can also see that I’m getting an error on the same line:
if (currentLoop instanceof MultiStairHandler) {
currentLoop.addResponse(SAM_1_instructions_key.corr, level)
https://drive.google.com/drive/folders/1teQWNXNcvlxpIVoHCTfrk2y6xAu7A4dB?usp=sharing
were you able to figure it out?