NullReferenceException: Object reference not set to an instance of an object?

I have a really annoying bug in my code and I can't for the life of me figure out whats wrong with it. any help will be much appreciated, Thank you in advance. please appreciate that I am very new to unity scripting. Heres my script...

 #pragma strict
 var isDead : boolean;
 var showDeadGUI : boolean;
 var canMove : boolean;
 var lastPositionY : float = 0f;
 var fallDistance : float = 0f;
 var player : Transform;
 
 private var controller : CharacterController;
 
 var currentHealth : float = 10.0f;
 
 
 function Start () {
 controller = GameObject.Find("First Person Controller").GetComponent(CharacterController); //require the first person controller script
 isDead = false; //is the player dead?
 showDeadGUI = false; //should be asking to restart?
 canMove = true; //can the player move?
 }
 
 function Update () {
 if(lastPositionY > player.transform.position.y) //last y position > player y
     {
         fallDistance += lastPositionY - player.transform.position.y; //if the last y position is more or equel to the fall distance change the players y position
     }
     
     lastPositionY = player.transform.position.y; //sets the last y position
     
     if(fallDistance >= 5 && controller.isGrounded) //defines the fall height
     {
         currentHealth -= 5; //takes 5 away from the players current health
         ApplyNormal(); //applys a normal statement
     }
      
     if(fallDistance <= 5 && controller.isGrounded) //if fall distance is less than 5 apply a normal 
     {
         ApplyNormal(); //applys a normal
     }
 if(currentHealth <= 0) //if the players health is less than or the same as 0 set isDead to true
 {
 isDead = true; //sets is dead to true
 }
 
 if (isDead == true) //is isDead true?
 {
 showDeadGUI = true; //should the code be showing Dead GUI
 canMove = false; //can the player move?
 {if (Input.GetKeyDown (KeyCode.Return)) {  //if player wants to restart press enter
     Application.LoadLevel (0);  //restart the game, the first level
     showDeadGUI = false; //once enter pressed stop showing dead GUI
             }
         }
     }
 if (canMove == false) //if the game is over we need to tell the script to freeze the game
 {
 Time.timeScale = 0.00; //sets the speed of the game to 0 FPS
     if (Input.GetKeyDown (KeyCode.Return)) //is the return key down?
         {
             Time.timeScale = 1.00; //sets the game to full playback rate
         }
     }
 }
 
 
 
 function ApplyNormal() //applys a normal
 {
         fallDistance = 0; //defines the fall distance
         lastPositionY = 0; //defines the fall height 
 }
 
 function OnGUI() //defines a GUI function
 {
     GUI.Box(Rect(10, 20, 100, 20),"" + currentHealth); //draws a box with current health
     if (isDead == false) //isDead false?
     {
     GUI.color = Color.red; //sets the color to red if false
     }else
     {
     GUI.color = Color.green; //sets the color to green if true
     }
     GUI.Box(Rect(10, 40, 100, 20),"" + isDead); //draws the isDead GUI box
     if (showDeadGUI == true)
     {
         GUI.backgroundColor = Color.red; //sets the dead GUIs background to red
         GUI.color = Color.red; //sets the text color to white
         GUI.Box(Rect(100, 400, 1000, 100),"" + "Press Enter To Restart. Don't feel bad that you died just press enter and pretend that it never happend"); //draws the dead GUI box and asks for a restart
     }
 }