[Map-Swap] `canGoBack` broken afterwards

[hffmnn]

Steps:

1. Login (triggers a map swap)
2. Navigate to a subview
3. Logout (triggers a map swap)
4. Login (triggers a map swap)

Expected: Routemaster.of(context).history.canGoBack returns false and no back button is visible.

Experienced: Routemaster.of(context).history.canGoBack returns true, the back button is visible. Pressing the back button does nothing (Routemaster.of(context).pop() triggers no navigation).

Video

https://user-images.githubusercontent.com/352753/142463424-68180862-aa96-4c81-a1aa-778af61c7269.mov