Warnings when using DialogButtonBox

import QtQuick 2.9
import QtQuick.Window 2.2
import QtQuick.Controls 2.3

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    Dialog {
        id: discardChangesDialog
        objectName: "discardChangesDialog"
        x: Math.round(parent.width - width) / 2
        y: Math.round(parent.height - height) / 2
        title: qsTr("Unsaved changes")
        modal: true

        Label {
            text: qsTr("The action you're about to perform could discard changes.\n\nContinue anyway?")
        }

        // Using a DialogButtonBox allows us to assign objectNames to the buttons,
        // which makes it possible to test them.
        footer: DialogButtonBox {
            Button {
                objectName: "saveChangesDialogButton"
                text: qsTr("Save")
                DialogButtonBox.buttonRole: DialogButtonBox.AcceptRole
            }
            Button {
                objectName: "discardChangesDialogButton"
                text: qsTr("Discard")
                DialogButtonBox.buttonRole: DialogButtonBox.DestructiveRole
            }
        }
    }
}