Assertion failure when using TapHandler

import QtQuick 2.12
import QtQuick.Layouts 1.12
import QtQuick.Controls 2.5

ApplicationWindow {
    id: window
    width: 400
    height: 400
    visible: true

    StackLayout {
        id: stackLayout
        anchors.fill: parent

        Rectangle {
            color: "#444"
        }
        Rectangle {
            color: "#666"
        }
    }

    TapHandler {}
//    TapHandler {
//        target: window.contentItem
//        onTapped: stackLayout.currentIndex = stackLayout.currentIndex == 0 ? 1 : 0
//    }
}