Reproducer attached. It's nothing more than a template project whose App.qml has an ApplicationWindow as root element instead of a plain Window. Also "import QtQuick.Controls" is added to properly expose ApplicationWindow.

Despite that project can run properly, in 2D View QDS complains about that ApplicationWindow is not supported as root element:

Don't quite understand why. As far as I concern, if a Window works then an ApplicationWindow should too.