A field must not exist in the database, but it must exist in the DCA

I remember we have added security checks for the file and page widgets so that a user cannot access fields that do not exist. However, the check for if a database field exists is inappropriate, we must check the DCA.

Background/Reason: I have a pageTree widget in Isotope where I store the values in a subtable (using `load_callback` and `save_callback`). I have also set the DCA to `doNotSaveEmpty=true` so I do not need a real database field in the table.