Fixes #1977685. Prevent crash when the soft keyboard is brought up.

ViewRoot keeps a reference to the previously focused View and use this reference
when trying to scroll the window to the focused rect. There situations however
in which the previously focused view might have been removed from the view
hierarchy. This happens on Home for instance when you trash an icon, or with
ListView's items. This change ensures the previously focused view is still part
of the view hierarchy.