Opening Design mode opens three new windows

1. Have a Qt version's bin directory in PATH.
   I can reproduce the issue with the official release of Qt 5.14.1 as well as my self-built Qt 5.15.2.
2. Start Qt Creator with empty settings but load the QmlDesigner plugin, e.g.:

   ~/qtcreator-6.0.0-beta2o/bin/qtcreator -tcs -load QmlDesigner
   

3. Create a new Qt Quick Application project.
   Use the wizard's defaults. I can reproduce the issue with projects using qmake as well as CMake.
   Creator will automatically open the project's main.qml.
4. Start the Design mode.
   Three windows will pop up which use the Window item's title from main.qml but are empty otherwise:
   

This happens with Qt Creator built on Qt 6.2.0 as well as Qt 6.2.1. With the same revision of Qt Creator built on Qt 5.15.2, I don't see the issue.