ScreenViewport issue

package atotb;

import com.badlogic.gdx.Game;
import com.badlogic.gdx.Screen;
import com.badlogic.gdx.graphics.Texture;
import com.badlogic.gdx.graphics.g2d.SpriteBatch;

public class TwoBrothersGame extends Game {
    SpriteBatch batch;
    Texture img;
    Screen gameScreen;

    @Override
    public void create () {
        batch = new SpriteBatch();
        img = new Texture("badlogic.jpg");
        gameScreen = new GameScreen(this);
        setScreen(gameScreen);
    }

    @Override
    public void dispose() {
        batch.dispose();
        img.dispose();
        gameScreen.dispose();
    }
}

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package atotb;

import com.badlogic.gdx.Gdx;
import com.badlogic.gdx.Screen;
import com.badlogic.gdx.graphics.GL20;
import com.badlogic.gdx.graphics.OrthographicCamera;
import com.badlogic.gdx.utils.viewport.ScreenViewport;
import com.badlogic.gdx.utils.viewport.Viewport;

class GameScreen implements Screen {
    TwoBrothersGame game;
    OrthographicCamera camera;
    Viewport viewport;

    public GameScreen(TwoBrothersGame game) {
        this.game = game;
        camera = new OrthographicCamera();
        viewport = new ScreenViewport(camera);
    }

    @Override
    public void render(float dt) {
        camera.update();
        Gdx.gl.glClearColor(1, 0, 0, 1);
        Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);
        game.batch.begin();
        game.batch.draw(game.img, 0, 0);
        game.batch.end();
    }

    @Override
    public void resize(int width, int height) {
        viewport.update(width, height);
    }

    @Override
    public void show() {
    }

    @Override
    public void hide() {
    }

    @Override
    public void pause() {
    }

    @Override
    public void resume() {
    }

    @Override
    public void dispose() {
    }
}