Applications: Closing subpage puts sidebar in invalid state

If the user closes out of the applications list subpage, an app detail page is paired up w/ the toplevel sidebar. See video:

Steps to reproduce:

Open GNOME Settings (version: aa0adcaa)
Open the applications page
Hit the back button in the sidebar
Observe bug

Alternatively:

Make the window small enough to fold the leaflet
Open the applications page
Select an application
Hit the back button twice
Make the window bigger to unfold the leaflet
Observe bug