problem with FindWithTag

Hello everyone I need some help. For you will be easy to fix but I'm trying in every possible way not accomplish much. What I want to do are these Christmas balls that fall from the top tray to random (and up to far so good) also these balls (made ​​up of 3 components: 1 ball and 2 hooks) change color every second and I have to click with your mouse over the red ball to gain points. At first I made ​​two scripts: the first script was connected to the chamber and was used to instantiate objects. The second script was used to change color at random and was linked to item 1 of the ball (the ball). I found myself in trouble, however, to pass the array of textures in the main room. I finished the idea in this way:

var speed : int = 2; var chrismasBall : GameObject; //GameObject var instanza; //instanza var texBall : Texture[] = new Texture [5]; //Array di texture

function Start () {

StartCoroutine(Creation(1)); //Coroutine

var finds = GameObject.Find("snow_ball"); // trova tag e assegno a finds Debug.Log(finds); // stampo l'oggetto in console

instanza.GetComponent(main).textureRandom();

}

function Update () { Physics.gravity = Vector3(0, -0.05, 0); // aggiungo gravità

}

function Creation(tempo){

       while(true){
   
   
   
    instanza = Instantiate (chrismasBall, transform.position, Quaternion.identity); //instanza chrismasBall
  
   
   
  instanza.AddComponent(Rigidbody); //aggiungo rigidbody alle instanze
  
   
   instanza.transform.position.x = Random.Range(2,-2); //raggio di instanza
   instanza.transform.position.y = 2; 
   instanza.transform.position.z = 0.2; //distanza dalla Main Camera
   instanza.transform.Translate(0, Time.deltaTime, 0); 
   
   yield WaitForSeconds(tempo);
   
   
   

}

}

function textureRandom(){

while(true){

 yield WaitForSeconds(1); //aspetto 1 secondo e cambio colore alla mesh
 
 var randomtex; // variabile randomColor
 
 randomtex = texBall[Random.Range(0, texBall.length)]; //random della variabile cambiaColore
 
   

renderer.material.mainTexture= randomtex; // lo assegno alla variabile randomColor

} }

But me error on line 15 saying:

Object reference not set to an instance of an object

Someone is kind enough to help me? thank you