APP Stopping to work

Question

What’s wrong with the code? In the IDE it appears that there are no errors but when I open the app it closes and appears the message "app stopped".

 public class BallControl extends Activity implements SensorEventListener {
    protected Sensor s;

    private float[] gravity = new float[] {0, 0};
    private BallControlView ballView;
    private float sensorX, sensorY;

    @Override
    protected void onCreate(Bundle savedInstanceState){
        super.onCreate(savedInstanceState);


        SensorManager sm = (SensorManager) getSystemService (Context.SENSOR_SERVICE);
        s = sm.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);

        if(s != null){
            ballView = new BallControlView(this);
            int ballView = 0;
            setContentView(ballView);
        }
    }

    @Override
    protected void onResume(){
        ballView.resume();
        super.onResume();}

    @Override
    protected void onPause(){
        ballView.pause();
        super.onPause();}

    @Override
    public void onSensorChanged(SensorEvent sensorEvent){

    }

    @Override
    public void onAccuracyChanged(Sensor sensor, int i){

    }

    public class BallControlView extends SurfaceView implements Runnable{
        Thread thread = null;

        Bitmap ball;
        SurfaceHolder surface;
        float speedX = 0, speedY = 0;
        float positionX =0, positionY =0;
        float mass;
        final float maxSpeed= 40f;
        final double maxSpeedXY = Math.sqrt(2) * maxSpeed;
        boolean enableSoundFX;

        MediaPlayer mp;

        boolean isRunning;

        public BallControlView (Context context){
            super(context); 

            setKeepScreenOn(true);
            ball = BitmapFactory.decodeResource(getResources(), R.drawable.ic_launcher);
            surface = getHolder();
            mp = MediaPlayer.create(context, R.raw.drum_0); }

        @Override
        public void run(){
            while(isRunning){
                if(surface.getSurface().isValid()){

                    Canvas canvas = surface.lockCanvas();
                    if(canvas != null){

                        float validWidth = getWidth() - ball.getWidth();
                        float validHeigth = getHeight() - ball.getHeight();

                        enableSoundFX = true;
                        mass = 30;

                        boolean collided = false;

                        speedX += sensorX/mass;
                        speedX = constrain(-maxSpeed, speedX, maxSpeed);
                        positionX = positionX + speedX;

                        if(positionX <= 0 || positionX >= validWidth) {
                            speedX = - .8f * speedX;
                            collided = true;}

                        positionX = constrain(0, positionX, validWidth);
                        speedY += sensorY/mass;
                        speedY = constrain(-maxSpeed, speedY, maxSpeed);
                        positionY = positionY + speedY;

                        if(positionY <= 0 || positionY >= validHeigth) {
                            speedY = - .8f * speedY;
                            collided = true;}

                        positionY = constrain(0, positionY, validWidth);
                        canvas.drawColor(Color.BLACK);
                        canvas.drawBitmap(ball, (int) positionX, (int) positionY, null);
                        surface.unlockCanvasAndPost(canvas);

                        if(enableSoundFX && collided){
                            float volume = (float)(Math.pow(Math.hypot((double) speedX, (double) speedY)/maxSpeedXY, 2));
                            mp.setVolume(volume, volume);

                            if(mp.isPlaying())
                                mp.seekTo(0);

                            else
                                mp.start();
                        }
                    }
                }
            }
        }


        private float constrain(float min, float value, float max){
            return Math.max(Math.min(value, max), min);}
        public void resume(){

            isRunning = true;
            thread = new Thread (this);
            thread.start();}

        public void pause(){
            isRunning = false;

            try{
                thread.join();} 
            catch(InterruptedException e){
                e.printStackTrace();}
            finally{
                thread = null;
            }
        }
    }
}