QML engine rootObjects still empty when Component.onCompleted is invoked

[Tom asso]

Why is the app engine's rootObject list still empty even when Component.onCompleted is invoked?

I want to call a C++ singleton backend method from qml when the engine has completely loaded the qml, without any user interaction. The backend method needs to access a specific qml item, so the method takes an objectName as input parameter, then invokes rootObject->findChild(objectName) to get the item's object. The qml looks like this:

Window {
   [...]
  Surface3D {
     Surface3DSeries {
       objectName = "myseries"
     }
  }
  // Invoke BackEnd C++ method when Window is complete
  Component.onCompleted: {
    BackEnd.foo("myseries");
  }
}

BackEnd.foo() looks like this:

bool BackEnd::foo() {
  char *seriesName = seriesNameString.toLatin1().data();
  if (g_appEngine->rootObjects().isEmpty()) {
      // rootObjects are empty - how to find seriesName object???
      qDebug("rootObjects are empty");
      return false;
    }
    // Get root window
  QObject *root = qobject_cast<QQuickWindow*>(g_appEngine->rootObjects().value(0));

 QSurface3DSeries *surface3DSeries = 
       root->findChild<QSurface3DSeries *>(seriesName);
  if (!surface3DSeries) {
      qCritical() << "Couldn't find QSurface3DSeries " << seriesNameString << " object";
      return false;
    }

So how can I invoke BackEnd.foo() from qml and find the object specified by objectName?

Thanks
Tom