App Preview version 2.3 has an issue with launching a new app. When you create a NEW app and try to test with App Preview (either through QR code or app list) it does not launch the app. The app list is not refreshed and hence the app will not launch. Workaround is :
1) Logout of the App Preview and log back in. This will refresh the app list and new app will be visible or scannable through QR code.
2) Close the App Preview app (remove from active apps) and then relaunch.
This will be fixed in the next release of App Preview.