@Override
    public void initialize(AppStateManager stateManager, Application app) {
        if (rootNode == null) {
            rootNode = new Node("Root Node");
        }
        if (viewPort == null) {
            viewPort = app.getViewPort();
        }
        viewPort.attachScene(rootNode);
        super.initialize(stateManager, app);
    }
Creates the AppState with the given unique ID, ViewPort, and root Node, attaches
the root Node to the ViewPort and updates it.

@param id the desired AppState ID
@param viewPort An existing ViewPort
@param rootNode An existing root Node
jme3-core/src/main/java/com/jme3/app/state/RootNodeAppState.java