This means that you are trying to read from an object that is not there. My assumption is that you got this error in a microflow? Set a breakpoint in the microflow and double check if all objects are there and change the code so that you always take the scenario into account that an object is not there.
Hi Ronald thank you for your reply!
I solved it now, the error was probably because I had a page with many conditional visibility rules (regarding booleand value of one attribute) and whenever I was executing a microflow on my page, I was changing this boolean value… when I deleted the conditional visibilities (I set them differently not using the boolean attribute) error was gone.