NullPointerException? Not sure why. LIBGDX

Question

I keep getting a NullPointerException on line 39, which is modelBatch.begin(cam);

I have no idea as to why it's doing this. If you notice why, please tell me. I've been struggling with this problem all day now. I am new to android development, and am prone to making silly mistakes. Thank you for your help.

public class Loading implements Screen {

private boolean AP;
private Chemistry chemistry;
public PerspectiveCamera cam;
public ModelBatch modelBatch;
public Model model;
public ModelInstance instance;
public Lights lights;

public Loading(boolean AP, Chemistry chemistry) {
    this.AP = AP;
    this.chemistry = chemistry;
}

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

    modelBatch.begin(cam);
    modelBatch.render(instance, lights);
    modelBatch.end();
}

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

@Override
public void show() {
            modelBatch = new ModelBatch();
    lights = new Lights();
    lights.ambientLight.set(0.4f, 0.4f, 0.4f, 1f);
    lights.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f,
            -0.2f));

    cam = new PerspectiveCamera(70, Gdx.graphics.getWidth(),
            Gdx.graphics.getHeight());
    cam.position.set(10f, 10f, 10f);
    cam.lookAt(0, 0, 0);
    cam.near = 0.1f;
    cam.far = 300f;
    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);

}
       ...
}

Answer 1

Move this to your constructor:

        modelBatch = new ModelBatch();
lights = new Lights();
lights.ambientLight.set(0.4f, 0.4f, 0.4f, 1f);
lights.add(new DirectionalLight().set(0.8f, 0.8f, 0.8f, -1f, -0.8f,
        -0.2f));

cam = new PerspectiveCamera(70, Gdx.graphics.getWidth(),
        Gdx.graphics.getHeight());
cam.position.set(10f, 10f, 10f);
cam.lookAt(0, 0, 0);
cam.near = 0.1f;
cam.far = 300f;
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);

like:

public Loading(boolean AP, Chemistry chemistry) {
    this.AP = AP;
    this.chemistry = chemistry;
    //here
}