DialogButtonBox cannot override standardButtons set in separate QML file

import QtQuick.Controls 2.1

Dialog {
    standardButtons: Dialog.Ok | Dialog.Cancel

    Label {
        text: "Test text"
    }
}

import QtQuick 2.7
import QtQuick.Controls 2.1

ApplicationWindow {
    visible: true
    width: 640
    height: 480

    Button {
        anchors.centerIn: parent
        text: "Open dialog"
        onClicked: dialog.open()

        CustomDialog {
            id: dialog

            x: (parent.width - width) / 2
            y: (parent.height - height) / 2
//            standardButtons: Dialog.NoButton

            footer: DialogButtonBox {
                standardButtons: DialogButtonBox.Save | DialogButtonBox.Cancel
            }
        }
    }
}