ApplicationWindow attached property not available in Popup

import QtQuick 2.6
import Qt.labs.controls 1.0

ApplicationWindow {
    width: 300;
    height:300
    visible: true

    Item {
        Component.onCompleted: console.log("item:", ApplicationWindow.window)
    }

    Popup {
        Component.onCompleted: console.log("popup", ApplicationWindow.window)

        Rectangle {
            Component.onCompleted: console.log("popup content", ApplicationWindow.window)
        }
    }
}