I have just updated the Qt Creator to 6.0.0 and the QML-Designer is not available anymore.
I have opened a project, created with Qt Creator 5.0.3, and after the update is not possible to edit the form:
The installed version of Qt is 5.15.2.
How to open *.ui.qml files?
Looks like they're disabling it to try and steer people to Qt Design Studio.
"The integrated Qt Quick Designer is now disabled by default. Qt Creator will open .ui.qml files in Qt Design Studio. This is a step towards a more integrated workflow between Qt Design Studio and Qt Creator (video). Qt Quick Designer is still there, you can manually enable it again by checking the QmlDesigner plugin in Help > About Plugins." from here
