Window{} defined in mail.qml is not displayed in design mode

Steps to reproduce:

1. In Qt Creator go to File --> New Project.…

2. Select Application (Qt for Python) --> Qt Quick Application - Empty --> Choose

3. Confirm Location

4. Define Project Details: PySide 6, Python 3.12, Create new virtual environment --> Continue

Click Done

Now main.qml opens, containing

import QtQuick
import QtQuick.WindowWindow {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
}

5. Select View --> Views --> 2D.

The Window is not visible in 2D View. Only a small blue rectangle saying „Window“ is shown. See Attachment.