ubuntu-ui-toolkit-launcher doesn't support Window as root item

Using a QtQuick.Window as a root item with ubuntu-ui-toolkit-launcher results in an error loading the QML:

import QtQuick 2.4
import QtQuick.Window 2.2
import Ubuntu.Components 1.3
Window {
    title: "Hello World"
    minimumWidth: units.gu(30)
    minimumHeight: units.gu(50)
    maximumWidth: units.gu(90)
    maximumHeight: units.gu(120)
    MainView {
        applicationName: "Hello World"
    }
}

QQuickView only supports loading of root objects that derive from QQuickItem.
If your example is using QML 2, (such as qmlscene) and the .qml file you
loaded has 'import QtQuick 1.0' or Qt 4.7', this error will occur.
To load files with 'import QtQuick 1.0' or Qt 4.7', use the
QDeclarativeView class in the Qt Quick 1 module.

That same piece of code works with qmlscene.

See also bug 1587431.