How to change button name in runtime?

Troubleshooting gui
1

I have following code:
[java]
package mygame;

import com.jme3.app.SimpleApplication;
import com.jme3.niftygui.NiftyJmeDisplay;
import de.lessvoid.nifty.Nifty;
import de.lessvoid.nifty.builder.LayerBuilder;
import de.lessvoid.nifty.builder.PanelBuilder;
import de.lessvoid.nifty.builder.ScreenBuilder;
import de.lessvoid.nifty.controls.button.builder.ButtonBuilder;
import de.lessvoid.nifty.elements.Element;
import de.lessvoid.nifty.elements.render.TextRenderer;
import de.lessvoid.nifty.screen.DefaultScreenController;

/**

  • @author iamcreasy
    */
    public class Main extends SimpleApplication {

    public static void main(String[] args) {
    Main app = new Main();
    app.start();
    }
    Nifty nifty;

    @Override
    public void simpleInitApp() { // make the cube appear in the scene
    NiftyJmeDisplay niftyDisplay = new NiftyJmeDisplay(
    assetManager, inputManager, audioRenderer, guiViewPort);
    nifty = niftyDisplay.getNifty();
    guiViewPort.addProcessor(niftyDisplay);
    flyCam.setDragToRotate(true);
    nifty.loadStyleFile(“nifty-default-styles.xml”);
    nifty.loadControlFile(“nifty-default-controls.xml”);
    guiFont = assetManager.loadFont(“Fonts/aaa.fnt”);
    nifty.addScreen(“Screen_ID”, new ScreenBuilder(“Hello Nifty Screen”) {
    {
    controller(new DefaultScreenController()); // Screen properties

             // <layer>
         layer(new LayerBuilder("Layer_ID") {
             {
                 childLayoutVertical(); // layer properties, add more...

                 // <panel>
                 panel(new PanelBuilder("Panel_ID") {
                     {
                         childLayoutCenter(); // panel properties, add more... 

                         // GUI elements
                         control(new ButtonBuilder("Button_ID", "Hello Nifty") {
                             {
                                 alignCenter();
                                 valignCenter();
                                 height("5%");
                                 width("15%");
                             }
                         });

                     }
                 });
                 // </panel>
             }
         });
         // </layer>
     }
 }.build(nifty));
 // </screen>

 nifty.gotoScreen("Screen_ID"); // start the screen
 Element niftyElement = nifty.getCurrentScreen().findElementByName("Button_ID");
 niftyElement.getRenderer(TextRenderer.class).setText("hello world!");

    }
    }
    [/java]

it got
SEVERE: Uncaught exception thrown in Thread[LWJGL Renderer Thread,5,main]
java.lang.NullPointerException
at mygame.Main.simpleInitApp(Main.java:71)
at com.jme3.app.SimpleApplication.initialize(SimpleApplication.java:226)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.initInThread(LwjglAbstractDisplay.java:130)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:207)
at java.lang.Thread.run(Thread.java:662)

2

problem is solved
It is the code
[java]
Element niftyElement = nifty.getCurrentScreen().findElementByName(“Button_ID”);
niftyElement.getNiftyControl(Button.class).setText(“hello”); [/java]

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