We recently had a business unit that was trying to make a very simple exploratory map for the general public. They wanted the popups for the layers to only display the title of the layer as that was the all that was relevant - but deleting the fields list would not actually save upon exiting the application. Mapviewer would appear to take the change, allow a save but then if you opened the map back up the fields list would be present again.
Our work around solution was to delete all the fields in the field list and that did work. However the lack of error messaging is a big issue here. Deleting the fields list appeared to work and was an allowed action. If it's not allowed then you shouldn't let users do it. Having to delete all the fields in a list is also a bit more time consuming and it would be nice if this delete was an option.
Encountered this a few weeks ago and again just now. Esri recognizes this behavior as BUG-000142581 (https://support.esri.com/en-us/bug/deleting-the-fields-list-section-from-the-popup-configu-bug-00014...).
Evidently the popup needs at least one element at all times. I also just removed all the fields from the fields list as a solution. Esris suggest using an empty text item to the same effect. Posting this for anybody that stumbles across this bug.