Qt Quick 2 cannot co-exist with widgets

Because QQuickCanvas is a QWindow, you can no longer embed QML scenes into QWidget UIs. You could do this with QtQuick 1, because QDeclarativeView was a QWidget. It would make sense to either add a QQuickCanvasWidget in QtWidgets, or solve this more generally with a QWindowWidget which paints and resizes a QWindow surface inside a QWidget (so you can put it in layouts and inside other QWidgets)