Neil, thank you so much for answering. I deleted and reinstalled the app and now the content is visible. It seems that I have to delete and reinstall every time I change something in cards and layouts. I'll just keep doing so while waiting for the release next week that Nathan mentioned, and if it doesn't work properly by then I'll start a new thread with screenshot.

I have the same problem, the preview app is not showing anything. It worked fine when I still had the default layout, but after I created cards and layouts it is just showing a blank white screen. I have used the same layouts, cards and grid as in the tutorial video. Anyone who knows what might be wrong?