player Randomly dies and error message appears.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.SceneManagement;

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

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

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

        // If the game is not started and the player presses the spacebar...
        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();

            // ...incrememt 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 the 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);
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerHealth : MonoBehaviour
{

    public bool alive;
    [SerializeField]
    private GameObject pickupPrefab;


    // Use this for initialization
    void Start()
    {
        alive = true;
    }

    void OnTriggerEnter(Collider other)
    {
        // ...If the collider other is tagged with player...
        // ...aka if the bird runs into the player...
        if (other.CompareTag("Enemy") && alive == true)
        {
            alive = false;

            // Create the pickup particles
            Instantiate(pickupPrefab, transform.position, Quaternion.identity);

            //// ...decrease total number of flies...
            //FlySpawner.totalFlies = 0;

            //// ...update the score...
            //ScoreCounter.score = 0;

            //Destroy(gameObject);
        }
    }
}