I'm just getting started with libGDX and have run into a snag registering an InputListener for a button. I've gone through many examples and this code appears correct to me but the associated callback never triggers ("touched" is not printed to console). I'm just posting the code with the abstract game screen and the implementing screen. The application starts successfully with a label of "Exit" in the bottom left hand corner, but clicking the button/label does nothing.

I'm guessing the fix is something simple. What am I overlooking?

public abstract class GameScreen<T> implements Screen
{
    protected final T game;
    protected final SpriteBatch batch;
    protected final Stage stage;

    public GameScreen(T game) 
    {
        this.game = game;
        this.batch = new SpriteBatch();
        this.stage = new Stage(0, 0, true);
    }

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

        // Clear the screen with the given RGB color (black)
        Gdx.gl.glClearColor(0f, 0f, 0f, 1f);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

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

    public abstract void update(float delta);

    @Override
    public void resize(int width, int height) 
    {
        stage.setViewport(width, height, true);
    }

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

    // hide, pause, resume, dipose
}

public class ExampleScreen extends GameScreen<MyGame>
{
    private TextButton exitButton;

    public ExampleScreen(MyGame game)
    {
        super(game);
    }

    @Override
    public void show()
    {
        super.show();

        TextButton.TextButtonStyle buttonStyle = new TextButton.TextButtonStyle();          
        buttonStyle.font = Font.getFont("Origicide", 32);
        buttonStyle.fontColor = Color.WHITE;

        exitButton = new TextButton("Exit", buttonStyle);
        exitButton.addListener(new InputListener() {
            @Override
            public void touchUp (InputEvent event, float x, float y, int pointer, int button) {
                System.out.println("touched");
            }
        });
        stage.addActor(exitButton);
    }

    @Override
    public void update(float delta) 
    {    
    }
}