libgdx - stage was disabled when back button pressed

Question

I'm using libgdx 1.9.0 for game development. I have one stage in main screen that includes multiple buttons. I have another screen code that shows game objects. when the back button in android device is pressed the stage in main screen gets disabled and listener functions for buttons don't work. In Main Screen I have :

public class MainMenuScreen extends ScreenAdapter {
TextButton button_start; 
TextButtonStyle textButtonStyle;
BitmapFont font;

FitViewport viewp;
Stage stage;

public MainMenuScreen(IndependentArch game) {
    this.game = game;


    /* add GUI controls */
    stage = new Stage(new StretchViewport(game.SCREEN_WIDTH,
            game.SCREEN_Height), game.batcher);
    Gdx.input.setInputProcessor(stage);

    textButtonStyle = new TextButtonStyle();
    textButtonStyle.font = Assets.font_btitr;
    textButtonStyle.up = Assets.skin.getDrawable("button");
    textButtonStyle.down = Assets.skin.getDrawable("button-down");

    button_start = new TextButton(Reshape.doReshape_mirror("start"),
            textButtonStyle);
    button_start.setBounds(0, 0, 200, 50);
    button_start.setPosition(140, 255);

    button_about.addListener(new ChangeListener() {
        @Override
        public void changed(ChangeEvent event, Actor actor) {

             MyTextInputListener listener = new MyTextInputListener();
            Gdx.input.getTextInput(listener, "name", "",
             "please enter your name.");

        }
    });

    stage.addActor(button_start);

}

public void update() {
    if (Gdx.input.justTouched()) {
        stage.getCamera().unproject(
                touchPoint.set(Gdx.input.getX(), Gdx.input.getY(), 0));

    }
}

public void draw(float delta) {
    Gdx.gl.glClearColor(0.09f, 0.28f, 0.2f, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    game.batcher.disableBlending();
    game.batcher.begin();
    game.batcher.draw(Assets.backgroundRegion[0], 0, 0, game.SCREEN_WIDTH,
            game.SCREEN_Height);
    game.batcher.end();

    stage.act(delta);
    stage.draw();

    game.batcher.enableBlending();
    game.batcher.begin();


    game.batcher.draw(Assets.uiskin.findRegion("music"),
            game.SCREEN_WIDTH - 90, 10, 80, 80);

    game.batcher.end();
}

@Override
public void render(float delta) {
    update();
    draw(delta);
}} 

and in Game Screen I have :

 public class GameScreen implements Screen, InputProcessor {
OrthographicCamera guiCam;
    Vector3 touchPoint;
World world;
WorldRenderer renderer;

CameraController camController;
InputMultiplexer multiplexer;

GestureDetector gestureDetector;

/** for debug rendering **/

ShapeRenderer debugRenderer = new ShapeRenderer();

int lastScore;
String scoreString;
float totalGameTime;

GlyphLayout glyphLayout = new GlyphLayout();

public GameScreen(IndependentArch game) {
    this.game = game;

    Gdx.input.setCatchBackKey(true);


    state = GAME_READY;
    guiCam = new OrthographicCamera(game.SCREEN_WIDTH, game.SCREEN_Height);
    guiCam.position.set(game.SCREEN_WIDTH / 2, game.SCREEN_Height / 2, 0);
    touchPoint = new Vector3();

    world = new World();
    renderer = new WorldRenderer(game.batcher, world);
    Gdx.input.setInputProcessor(this);

    camController = new CameraController(world);
    multiplexer = new InputMultiplexer();
    multiplexer.addProcessor(new GestureDetector(20, 0.5f, 2, 0.15f,
            camController));
    multiplexer.addProcessor(this);
    Gdx.input.setInputProcessor(multiplexer);

}


// * InputProcessor methods ***************************//

@Override
public boolean keyDown(int keycode) {

    if (keycode == Keys.BACK) {
        game.setScreen(new MainMenuScreen(game));
        // Gdx.app.exit();
    }

    return false;
}}

Answer 1

Override the show() method in the main menu screen and set the input processor to stage there

@Override
public void show() {
    Gdx.input.setInputProcessor(stage);
}