For movie titles that don't have a backdrop to select in the database, the select poster and select backdrop choices result in opposite selection menus.

When you "select backdrop", the list of avaialble posters shows. When you "select poster", the list of avaiable backdrops shows, which is a blank screen, as there are no backdrops to select. On the initial poster and backdrop selection pop up menu there is no "open backdrop" option either, likely because there is no backdrop to display, but this does show in previous releases, and the result when selected is a blank screen as expected.

An example movie that you can test this on is "Stuart Saves His Family (1995)", IMDB tt0114571.