help with game error

I dont know what i am doing wrong? I get an error when running the game and when i cut down a tree i get ""NullReferenceException: Object reference not set to an instance of an object cutTree.Update ()""

This is my code: var trees: GameObject[]; var bullitPrefab:Transform;

 function Start() {
 
  trees = GameObject.FindGameObjectsWithTag("Tree"); 
 
  
 
 }
 
  function Update()
 
  
 
 {
 
  
 
  if(Input.GetButton("placeLog")) {
 
  
 
    for(var tree in trees) {
 
  
 
      if(tree == null)
 
  
 
        continue;
 
  
 
       
 
  
 
      if(Vector3.Distance(tree.transform.position, transform.position) < 5) 
 
  
 
      {
 
  
 
         Instantiate(bullitPrefab, GameObject.Find("logSpawn").transform.position, Quaternion.Identity);
 
  
 
         Destroy(tree);
 
  
 
         break;
 
  
 
      }
 
  
 
    }
 
  
 
  
 
  
 
  }
 
  
 
 }

could i have some help please