stackable-box: Check is visible child exists in folded mode
The assumption has always been that if the widget is folded, there's a visible child. That's true for leaflet, but not for deck, so we need a check to avoid a crash.
Fixes https://source.puri.sm/Librem5/libhandy/issues/243
Signed-off-by: Alexander Mikhaylenko alexm@gnome.org