Random Spawn Problem

So im having a code that let bombs randomly rain from the sky.

This is the code:

 var timer : float = 0.0;
 var spawning : boolean = false;
 var prefab : GameObject;
 var spawn1 : Transform;
 var spawn2 : Transform;
 var spawn3 : Transform;
 var spawn4 : Transform;
 var spawn5 : Transform;
  
 function Update () {
  //check if spawning at the moment, if not add to timer
  if(!spawning){
   timer += Time.deltaTime;
  }
  //when timer reaches 2 seconds, call Spawn function
  if(timer >= 2){
   Spawn();
  }
 }
  
 function Spawn(){
  //set spawning to true, to stop timer counting in the Update function
  spawning = true;
  //reset the timer to 0 so process can start over
  timer = 0;
  
  //select a random number, inside a maths function absolute command to ensure it is a whole number
  var randomPick : int = Mathf.Abs(Random.Range(1,7));
  
  //create a location 'Transform' type variable to store one of 3 possible locations declared at top of script
  var location : Transform;
  
  //check what randomPick is, and select one of the 3 locations, based on that number
  if(randomPick == 1){
   location = spawn1;
   Debug.Log("Chose pos 1");
  }
  else if(randomPick == 2){
   location = spawn2;
   Debug.Log("Chose pos 2");
  }
  else if(randomPick == 3){
   location = spawn3;
   Debug.Log("Chose pos 3");
  }
  else if(randomPick == 4){
   location = spawn4;
   Debug.Log("Chose pos 4");
  }
  else if(randomPick == 5){
   location = spawn5;
   Debug.Log("Chose Pos5");
   }
  
  //create the object at point of the location variable
 var  GameObject = Instantiate(prefab, location.position, location.rotation);
  
  //halt script for 1 second before returning to the start of the process
  yield WaitForSeconds(1);
  //set spawning back to false so timer may start again
  spawning = false;
 }

It works for a few seconds but then i got the error: NullReferenceException: Object reference not set to an instance of an object