if(assetManager.update()){
            game.setScreen(new MenuScreen(game, assetManager));
        }

ChangeListener listener = new ChangeListener() {
            @Override
            public void changed(ChangeEvent event, Actor actor) {

                //play button
                if(actor.equals(playBtn)) {
                    menuMusic.stop();
                    game.setScreen(new GameScreen(game, assetManager, settingsPrefs));
                }
           }
     }

E/AndroidRuntime: FATAL EXCEPTION: GLThread 4474
java.lang.NoClassDefFoundError: com.myname.mygame.worlds.MyContactListener
E/AndroidGraphics: waiting for pause synchronization took too long; assuming deadlock and killing

world = new World(0f, -10f);

FATAL EXCEPTION: GLThread 8739
        Process: com.myname.mygame, PID: 12794
java.lang.NoClassDefFoundError: Failed resolution of: Lcom/badlogic/gdx/physics/box2d/World;