Hey Grafeío Yardim, do you still get this error? And from the error itself, this seems to have been caused by Page Background field which then affected the World date from saving - currently in your world the Page Background (located in the Styling tab when editing your world) is empty, did you previously try to set an image to it? And if so, which one?
I didnt have anything in my sytling but I eventually changed the BOOSTRAP version to 5 and back to 3. After doing that it started working like normal again. I meant to close this report but got distracted.