import com.badlogic.gdx.ApplicationListener;
import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.InputAdapter;
import com.badlogic.gdx.InputMultiplexer;
import com.badlogic.gdx.graphics.Color;
import com.badlogic.gdx.graphics.GL10;
import com.badlogic.gdx.graphics.PerspectiveCamera;
import com.badlogic.gdx.graphics.VertexAttributes.Usage;
import com.badlogic.gdx.graphics.g3d.Environment;
import com.badlogic.gdx.graphics.g3d.Material;
import com.badlogic.gdx.graphics.g3d.Model;
import com.badlogic.gdx.graphics.g3d.ModelBatch;
import com.badlogic.gdx.graphics.g3d.ModelInstance;
import com.badlogic.gdx.graphics.g3d.attributes.ColorAttribute;
import com.badlogic.gdx.graphics.g3d.environment.DirectionalLight;
import com.badlogic.gdx.graphics.g3d.utils.ModelBuilder;

public class Game implements ApplicationListener {

    public PerspectiveCamera cam;
    public Model model;
    public ModelInstance instance;
    public ModelBatch modelBatch;
    public Environment environment;
    public InputMultiplexer input;

    public static int gWidth = 800;
    public static int gHeight = 600;

    @Override
    public void create() {

        this.modelBatch = new ModelBatch();

        this.environment = new Environment();
        this.environment.set(new ColorAttribute(ColorAttribute.AmbientLight, 0.4f, 0.4f, 0.4f, 1f));
        this.environment.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f, -0.2f));

        /**
         * Add input adapters.
         */
        this.cam = new PerspectiveCamera(67, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
        this.cam.position.set(10f, 20f, 0);
        this.cam.lookAt(0, 0, 0);
        this.cam.rotate(67f, 0f, 0f, 0f);
        this.cam.near = 1f;
        this.cam.far = 300f;

        this.input = new InputMultiplexer(Gdx.input.getInputProcessor());
        this.input.addProcessor(new MouseCamController());
        Gdx.input.setInputProcessor(this.input);

        this.cam.update();

        ModelBuilder modelBuilder = new ModelBuilder();
        model = modelBuilder.createBox(5f, 5f, 5f,
                new Material(ColorAttribute.createDiffuse(Color.GREEN)),
                Usage.Position | Usage.Normal);
        instance = new ModelInstance(model);
    }

    @Override
    public void resize(int width, int height) {
    }

    @Override
    public void render() {
        Gdx.gl.glViewport(0, 0, Gdx.graphics.getWidth(), Gdx.graphics.getHeight());
        Gdx.gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT);

//        if (this.cam.fieldOfView < 120) {
//            this.cam.fieldOfView += 10;
//        }
        this.cam.update();

        this.modelBatch.begin(this.cam);
        this.modelBatch.render(instance, environment);
        this.modelBatch.end();

    }

    @Override
    public void pause() {
    }

    @Override
    public void resume() {
    }

    @Override
    public void dispose() {
        this.modelBatch.dispose();
        this.model.dispose();
    }

    public class MouseCamController extends InputAdapter {
        @Override
        public boolean scrolled(int amount) {
//            Gdx.app.log("INPUT", "A: " + amount);
            return true;
        }
    }
}

Exception in thread "LWJGL Application" java.lang.NullPointerException
    at com.badlogic.gdx.InputMultiplexer.mouseMoved(InputMultiplexer.java:107)
    at com.badlogic.gdx.backends.lwjgl.LwjglInput.processEvents(LwjglInput.java:303)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication.mainLoop(LwjglApplication.java:200)
    at com.badlogic.gdx.backends.lwjgl.LwjglApplication$1.run(LwjglApplication.java:114)