"TypeError: Cannot read property of null" warning when closing application with a ButtonStyle component defined in a QML singleton

file:///home/mitch/dev/Qt5.5.0/5.5/gcc_64/qml/QtQuick/Controls/Styles/Base/ButtonStyle.qml:151: TypeError: Cannot read property of null

import QtQuick 2.3
import QtQuick.Window 2.2
import QtQuick.Controls 1.0

import "."

Window {
    visible: true

    Button {
        text: "hello!"
        anchors.centerIn: parent
        style: GameStyle.buttonStyle
    }
}

pragma Singleton

import QtQuick 2.4
import QtQuick.Controls 1.3
import QtQuick.Controls.Styles 1.3

Item {
    // produces warnings on exit when used
    property Component buttonStyle: ButtonStyle {
    }
}

singleton GameStyle 1.0 GameStyle.qml