REG: Binding to var properties that are undefined is broken in 6.2.0

import QtQuick 2.0
import QtQuick.Controls 2.0

ApplicationWindow {
    x: 800
    y: 400
    width: 400
    height: 400
    visible: true

    QtObject {
        id: theme

        property var toolButtonWidth: undefined
        property var toolButtonHeight: undefined
    }

    Button {
        id: root
        text: "Button"
        // Doesn't work with >= 6.0
        width: theme.toolButtonWidth
        height: theme.toolButtonHeight
        // Works
//        width: undefined
//        height: undefined
        anchors.centerIn: parent
        onClicked: print("clicked")
    }
}