Upgrading from Qt Creator 5.0 breaks existing Qt Quick projects

Core issue

Qt Creator now wants to open .ui.qml files in Qt Design Studio by default. However, this cannot work for existing projects because Qt Design Studio requires a .qmlproject file – this obviously isn't available in projects that pre-date Qt Design Studio.

Example of customer frustration
Comment from customer: "Why is Qt asking me for this? My projects have never needed a .qmlproject file before, and I don't know how to generate one. All of my projects are loaded through a .pro file. Additionally, I no longer have the option to create new ui.qml files"

This shows that the Principle of Least Astonishment (https://en.wikipedia.org/wiki/Principle_of_least_astonishment ) has not been followed.

Example project

Load the Coffee Machine example (https://doc.qt.io/qt-6/qtdoc-demos-coffee-example.html ) in Qt Creator with default settings. Trying to edit the example now produces an error with no clear indication on how to resolve the error.

Workaround

Re-enable the integrated Qt Quick Designer plugin in Qt Creator:

Help > About plugins... > Qt Quick > QmlDesigner

Suggestion

Qt Creator should check for the existence of a .qmlproject file. If it is not found, either fall back to the old integrated designer or advise the user on how to proceed.

Ideally, Qt Creator and/or Qt Design Studio should provide (semi-)automated help with generating of a .qmlproject file for existing projects.