Game error

using UnityEngine;
using System.Collections;
using UnityEngine.UI;

public class GameState : MonoBehaviour {

    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 game position
        followCamera.enabled = false;
    }

    // Update is called once per frame
    void Update () {
        if (gameStarted == false && Input.GetKeyUp (KeyCode.Space) || Input.GetKeyUp (KeyCode.Return) || Input.GetKeyUp (KeyCode.UpArrow)) {
            StartGame();
        }
        // if the player is no longer alive end game
        if (playerHealth.alive == false) {
            EndGame();

            //timer count up to restarting
            restartTimer = restartTimer + Time.deltaTime;
            // and if it reaches the restart delay..
            if (restartTimer >= restartDelay) {
                // ... then reload the currently loaded level
                Application.LoadLevel (Application.loadedLevel);
            }
        }
        if (Input.GetKey(KeyCode.Escape))
        {
            Application.Quit();
        }
    }
    private void StartGame() {
        // set the game state
        gameStarted = true;
        // remove the start text
        gameStateText.color = Color.clear;
        // allow the player to move
        playerMovement.enabled = true;
        birdMovement.enabled = true;
        followCamera.enabled = true;
    }
    private void EndGame() {
        gameStarted = false;
        // show the game over text
        gameStateText.color = Color.white;
        gameStateText.text = "You Lose!";
        // remove the player
        player.SetActive (false);
    }
}