Fix NPE when closing folder.

There is a race condition where Folder is closing, it unbinds, and then
another call to AbstractFloatingView.closeAllOpenViews tries to close the
Folder again. But since the contents are unbound and removed from the folder,
it produces a NPE.

Bug: 153625271
Change-Id: Ia6c55f15cf7517e26af8f499bc0c1a4828737480