Once you have selected a project page to be displayed in an i-frame via the Page List button, getting rid of it is not easily done. Re-selecting the "Choose from this list when..." option and then saving that change via the OK button reverts the setting back to the previously selected page. The only way to clear it is to use the Filepath button and then cancel that operation. That is not a very intuitive process.
Perhaps a setting of "None" as one of the page list options would be better.
I have found the correct way to deactivate the page list and reactivate the file path box. It is to un-set the 'Page List' button. That is, press it again so that it is not depressed. The clue was in the message 'Choose from this list when the Page link button is checked'. I agree it is not obvious but it is really easy once you know how.
Probably we should still add a 'none' at some point.