Overlay seems to be unknown to the typesystem

import QtQuick
import QtQuick.Controls.Material

Window {
    id: win
    width: 640
    height: 480
    visible: true
    title: "Sandbox"

    Button {
        onClicked: popup.open()

        Popup {
            id: popup

            parent: Overlay.overlay
  
            x: Math.round((parent.width - width) / 2)
            y: Math.round((parent.height - height) / 2)
            width: 100
            height: 100
       }
    }
}