iOS: Canvas draw black background only

import QtQuick 2.2
import QtQuick.Controls 1.1

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    Canvas {
        anchors.fill: parent
        onPaint: {
            var ctx = getContext('2d')
            ctx.save()
            ctx.beginPath();
            ctx.strokeStyle = '#FF0000'
            ctx.lineWidth = 5
            ctx.translate(50, 50)
            ctx.moveTo(0, 0)
            ctx.lineTo(500, 500)
            ctx.stroke()
            ctx.closePath()
            ctx.restore();
        }
    }

}