I have a qml6 Plasmoid. It is a simple calendar. When checking it’s configuration page is empty (it has only the frame). Here is the code, where am I wrong?

import QtQuick 2.0
import QtQuick.Layouts 1.1
import QtQuick.Controls 1.3
import org.kde.plasma.plasmoid 2.0
import org.kde.kirigami as Kirigami
import org.kde.plasma.core 2.0 as PlasmaCore
import org.kde.plasma.components 3.0 as PlasmaComponents

Item
{
  id: configRoot

  signal configurationChanged

  property alias cfg_widgetWidth:    widgetWidth.value

  ColumnLayout
  {

    RowLayout
    {
      Label { text: "Widget width" }
      SpinBox
      {
        id:           widgetWidth
        minimumValue: 10
        maximumValue: 10000
        decimals:     0
        stepSize:     10
        suffix:       ""
      }
    }
  }
}

I checked other plasmoids and documentation to no avail.