DialogButtonBox don't create a button when there is only one button.

import QtQuick 2.8
import QtQuick.Controls 2.1
import QtQuick.Layouts 1.0

ApplicationWindow {
    visible: true
    width: 320
    height: 240
    title: qsTr("Hello World")

    Popup {
        visible: true
        modal: true

        ColumnLayout {
            Label {
                text: "Message"
            }
            DialogButtonBox {
                standardButtons: DialogButtonBox.Ok         // NG
//                standardButtons: DialogButtonBox.Ok | DialogButtonBox.Cancel  //OK
            }
        }
    }
}

            DialogButtonBox {
                delegate: Button {
                    Component.onCompleted: console.debug(text)
                }

                standardButtons: DialogButtonBox.Ok         // NG
//                standardButtons: DialogButtonBox.Ok | DialogButtonBox.Cancel  //OK
            }