Repeater or GridLayout have problem with external property

import QtQuick 2.6
import QtQuick.Layouts 1.3
import QtQuick.Controls 2.2
import QtQuick.Window 2.0

ApplicationWindow {
    id: mainWindow
    visible: true

    property int baseSize: 20

    Component.onCompleted: app.quickLoadGame();
    RowLayout {
        Button { text: "+"; onClicked: repeater.model += 1; }
        Text   { text: repeater.model; }
        Button { text: "-"; onClicked: repeater.model -= 1; }

        GridLayout {
            columns: 8

            Repeater {
                id: repeater
                model: 16

                Rectangle {
                    color:  "red"
                    width:  baseSize
                    height: baseSize
                    // Layout.preferredWidth: baseSize
                    // Layout.preferredHeight: baseSize

                    Text { anchors.fill: parent; text: index; }
                }
            }
        }
    }
}