Open UI files in Qt Design Studio
By default, Qt Creator opens QML files (.qml) and UI files (.ui.qml) in a visual editor called Qt Quick Designer in the Design mode.
To open .ui.qml files in Qt Design Studio instead:
- Go to Preferences > Qt Quick > QML/JS Editing.

- In Open .ui.qml files with, select Qt Design Studio.
See also How to: Design Qt Quick UIs, Qt Quick UI design, Designing Qt Quick UIs, and Qt Design Studio documentation.