Mac: Warning while destroying a Menu

import QtQuick 2.1
import QtQuick.Controls 1.0

Item {
    id: root
    width: 640
    height: 480
    property bool created: false
    property var combobox;
    Button {
        text: qsTr("click me")
        anchors.centerIn: parent
        onClicked:  {
            if (!created) {
                combobox = Qt.createQmlObject('import QtQuick.Controls 1.0 ; ComboBox {}', root, '');
                created = true
            } else {
                combobox.destroy()
            }
        }
    }
}