carousel: Null check child when scrolling

Fix a crash when scrolling in an empty carousel.
6 jobs for main in 5 minutes and 13 seconds (queued for 9 seconds)
Status Name Job ID Coverage
  Build
passed abi-check #1572380

00:01:01

passed api-visibility #1572377

00:00:31

passed build-flatpak #1572379
flatpak

00:02:41

passed doc #1572378
flatpak

00:00:51

 
  Publish
passed refresh-doc #1572381

00:00:43

 
  Deploy
passed nightly #1572382

00:02:31