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 Librem5/libhandy#243

Signed-off-by: Alexander Mikhaylenko <alexm@gnome.org>
11 jobs for wip/exalm/deck-crash in 16 minutes and 26 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #259966
librem5
api-visibility

00:02:43

passed #259968
librem5
build-debian-clang

00:04:21

passed #259967
librem5
build-debian-gcc

00:03:01

passed #259969
librem5
build-fedora-gcc

00:03:21

 
  Test+Docs
passed #259970
librem5
build-gtkdoc

00:02:16

passed #259971
librem5
unit-test

00:02:12

43.4%
 
  Package
passed #259972
librem5
package-debian-buster

00:02:13

passed #259973
librem5:arm64
package-debian-buster:arm64

00:06:00

passed #259974
flatpak
package-flatpak

00:01:44

 
  Test Package
passed #259975
librem5
autopkgtest-debian-buster-package

00:02:02

passed #259976
librem5
lintian-debian-buster-package

00:00:47