1. Start Creator with an empty settings directory and without a Qt version in PATH.
2. Go to Welcome mode, "Examples" tab.
3. Click into the empty combo box.
   A white line will appear above and to the left of the combo box.
   
   On the command line Creator was called from, you can read:

   void QWindow::setTransientParent(QWindow*) QQuickView(0x28cb230, name = "WelcomePage") must be a top level window.
   

If such a line appears, it should be directly at the combo box.

If this is a Qt bug, please forward it and add missing information. I couldn't reproduce the issue in a minimal example.