NullReferenceException

Hi! Please help me with my code. I want to destroy “S3” and “C3” everytime I click “S3” at the same time I want to show “C4” bbut I always get NullReferenceExcepton error.

function Update () {

 if(Input.GetMouseButtonDown(0))
    {
       var hit: RaycastHit;
       var ray: Ray=Camera.main.ScreenPointToRay (Input.mousePosition);
       var C3: GameObject;
       C3 = GameObject.Find("C3");
       var S3: GameObject;
       S3 = GameObject.Find("S3");
       var C4: GameObject;
       C4 = GameObject.Find("C4");
          if(Physics.Raycast(ray, hit))
       {
	       	if(hit.transform.name == "S3")
	         {
	         C4.renderer.enabled=true;
	         Destroy(this.gameObject);
	         Destroy(S3);	 
	         print("hey");
	         
	         
	         }
	         }
	         }
	         }

thanks to those who will help me.

The problem seems to be that the C4 object is returning null. Here is a quick check to make sure it is not null before continuing:

if(hit.transform.name == "S3")
{
	// Execute everything you
	// need from this script

    // Make sure there is an object
    // called C4
    if(C4 != null)
    {
      Debug.Log("Found C4");
      C4.renderer.enabled = true;

      Destroy(S3);   
	  print("hey");
	
	  // Finally, destroy this
	  // gameObject
	  Destroy(this.gameObject);
    }    	
}

Hope that helps.