Error with UI

private bool gameStarted = false;
[SerializeField]
private Text gameStateText;
[SerializeField]
private GameObject player;
[SerializeField]
private BirdMovement birdMovement;
[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 = false;

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

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

    // Prevent the bird from moving at the start of the game
    birdMovement.enabled = false;

    //Prevent the follow camera from moving to its game position
    followCamera.enabled = false;
}

// Update is called once per frame
void Update () {
    // If the game is not started and the player presses the space bar...
    if (gameStarted == false && Input.GetKeyUp (KeyCode.Space)) {

        //...start the game.
        StartGame();
    }
    // If the player is no longer alive
    if (playerHealth.alive == false) {

        //...end the game...
        EndGame();

        //...increment a timer to count up to restarting...
        restartTimer = restartTimer + Time.deltaTime;

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

            //...then reload the currently loaded scene.
            SceneManager.LoadScene(SceneManager.GetActiveScene().buildIndex);

        }
    }
}


private void StartGame () {

    // Set the game state
    gameStarted = true;

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

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

    // Allow the bird to move
    birdMovement.enabled = true;

    // Allow the camera to move
    followCamera.enabled = true;



}

private void EndGame () {

    // Set the game state 
    gameStarted = false;

    // Show the game over text
    gameStateText.color = Color.white;
    gameStateText.Text = "Game Over!";

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

}