Assert on setting binding to alias property

import QtQuick 2.7
import QtQuick.Controls 2.0

ApplicationWindow {
    id: _window
    property string userFontFamily: "Times New Roman"

    Column {
        Button {
            id: _button
            text: "CLICK ME"
            onClicked: {
                _box.font.family = Qt.binding(function() { return _window.userFontFamily; });
            }
        }

        Rectangle {
            id: _box
            width: 100
            height: 100
            property alias font: _text.font

            Text {
                id: _text
                anchors.fill: parent
                text: "Text"
            }
        }
    }
}