Problem with Unity Ray Trace AI Detect Player

I am working on a police pursuit game. I have it working so the police car will see the player withing a radius in front of it but because I am using a collider it is detecting the player through walls and buildings. I'v tried using the stealth tutorials raycast code and can't really tell if its going to work or not but got this error:

 NullReferenceException: Object reference not set to an instance of an object
 PoliceVision.OnTriggerStay (UnityEngine.Collider other) (at Assets/Scripts/PoliceVision.js:27)

Am I going in the right direction? I've read a load of raycast AI questions but couldn't seem to get it working. Any help would be much appreciated!!

Here's "my" code:

 var isPlayerInSightRange : boolean = false;
 var playerInSight : boolean = false;
 
 private var col : SphereCollider;                       // Reference to the sphere collider trigger component.
 private var player : GameObject;                        // Reference to the player.
 
 public var fieldOfViewAngle : float = 110f;             // Number of degrees, centred on forward, for the police see.
 
     function OnTriggerStay (other : Collider)
      {
            if(other.gameObject.tag == "Player")
             {
             isPlayerInSightRange = true;
             playerInSight = false;
             print ("Player is within view distance but not visible.");
             
         var direction : Vector3 = other.transform.position - transform.position;
         var angle : float = Vector3.Angle(direction, transform.forward);
         
         // If the angle between forward and where the player is, is less than half the angle of view...
         if(angle < fieldOfViewAngle * 0.5f)
         {
         print ("debug1");
             var hit : RaycastHit;
             
             // ... and if a raycast towards the player hits something...
             if(Physics.Raycast(transform.position + transform.up, direction.normalized, hit, col.radius))
             {
             print ("debug2");
                 // ... and if the raycast hits the player...
                 if(hit.collider.gameObject == player)
                 {
                   print ("debug3");
                     // ... the player is in sight.
                     playerInSight = true;
                     // if playerInSight = true do stuff            
                     }
                  }
               }
          }
      
          else if (other.gameObject.tag == "checkpoint")
          {   
              // Police car will radio out its location
          }
      }
      
     function OnTriggerExit (other : Collider)
      {
            if(other.gameObject.tag == "Player")
             {
             isPlayerInSightRange = false;
             print ("Player is no longer within view distance.");
          }
      }