NullReferenceException

private bool gameStarted = false;
[SerializeField]
private Text gameStateText;
[SerializeField]
private GameObject player;
[SerializeField]
private BirdMovement birdMovment;
[SerializeField]
private FollowCamera followCamera;
private float restartDelay = 3f;
private float restartTimer;
private PlayerMovement playerMovement;
private PlayerHealth playerHealth;


// Use this for initialization
void Start () {
    Cursor.visible = true;

    playerMovement = player.GetComponent<PlayerMovement>();
    playerHealth = player.GetComponent<PlayerHealth>();

    //...prevent player from moving at start of game
    playerMovement.enabled = false;

    //.. prevent bird from moving at start of game
    birdMovment.enabled = false;

    //prevent follow camera from moving to its game posotion
    followCamera.enabled = false;


}



// Update is called once per frame
void Update () {

    // if the game is not starte and player presses the space bar..
    if(gameStarted == false && Input.GetKeyUp (KeyCode.Space)) {

        //... then start the game
        StartGame();

    }
    //if player is no longer alive.
    if (playerHealth.alive == false)  {

        //then end the game
        EndGame();

        //..increment a time to count up to estarting
        restartTimer = restartTimer + Time.deltaTime;

        //... and if it reaches the restart delay
        if (restartTimer >= restartDelay)   {

            //.. then reload the curently loaded level
            Application.LoadLevel(Application.loadedLevel);

        }

    }

}

private void StartGame(){
    //Set game state
    gameStarted = true;

    //remove the start text
    gameStateText.color = Color.clear;


    //allow player to move
    playerMovement.enabled = true;

    //allow bird to move
    birdMovment.enabled = true;

   // allow follow camera
    followCamera.enabled = true;
}

private void EndGame() {

    //set game state
    gameStarted = false;

    // show game over tet
    gameStateText.color = Color.white;
    gameStateText.text = "Game over!";

    //remove the player from game
    player.SetActive(false);

}