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

Questions & Answers
1

What is this error and why do i keep getting it? I am very new to coding please help. Here is my code :

#pragma strict
//Player Script

//Inspector Variables
var tagName             : String;

var rayDistance         : float = 100.0 ;  //length of ray for the raycast

var score               : int   = 0;       // Score for the player

var gameTime            : float = 20;       //The time the game will last 

var loadWaitTime        : float = 1.0;      //Amount of time to wait before next scene is loaded

var numberOfPointsToWin : int   = 5;

//Private Variables


function Start()
{
  InvokeRepeating("CountDown", 1.0, 1.0);        //Repeat countdown every second
}

//Update is called every frame
function Update () 
{

  
  if(Input.GetMouseButtonDown(0))
 {  
    
     var hit : RaycastHit;
     
     var ray : Ray =Camera.main.ScreenPointToRay(Input.mousePosition); //gets mouse position 
     
     if(Physics.Raycast(ray, hit, 100.0))
    
     {
            if (hit.transform.tag == tagName) {
                
                var enemyScript = hit.transform.GetComponent(ScriptActorEnemy);
                                
                enemyScript.numberOfClicks -= 1;                       //Reduce the number each click
                
                //Checks if object is at 0 for adding points to the score
                if(enemyScript.numberOfClicks == 0)
                {
                   score += enemyScript.enemyPoint;                    //adds points to overall score
                } 
                
                            
     }
    }
  }   
}


function CountDown()
{	if(--gameTime == 0)  //Subtract from gameTime
	{
	  CancelInvoke("CountDown");  //Cancel CountDown at 0
	  if(score >= numberOfPointsToWin)
	  {
	  	Application.LoadLevel("sceneScreenWin");
	  }
	  else
	  {
	    Application.LoadLevel("sceneScreenLose");
	  }
	}
	
}
//
function OnGUI()
{
	GUI.Label(Rect(10,10,100,20),"Score: "+ score);
	GUI.Label(Rect(10,25,100,35),"Time: "+ gameTime);
}
2

var enemyScript = hit.transform.GetComponent(ScriptActorEnemy);

should be

var enemyScript : ScriptActorEnemy = hit.transform.GetComponent(ScriptActorEnemy);

EDIT: Woops, typos. Corrected.