Accessing components through a collision

I don't like quoting errors, but I have no idea how to solve this one:

"NullReferenceException: Object reference not set to an instance of an object"

...followed by a whole load of bits that start with "Boo.Lang.Runtime.....etc"

All I am trying to do is set up an RTS-style selecting system where you click one something to select it. I have tried doing it several different ways (which is why the code below is a bit convaluted) but keep getting the same sort of error. I thought it was something to do with not accessing the right game object in the hierachy, but now I'm just confused.

My current workings:

In the main camera:

 var ray : Ray;
 var hit : RaycastHit;
 private var clickedObject;
 
 function Update () {
     if (Input.GetButton("Left Click")) {
         var layerMask = 1 << 8;
         layerMask = ~layerMask;
         ray = camera.main.ScreenPointToRay(Input.mousePosition);
         if (Physics.Raycast(ray,hit,100,layerMask)) {
             hit.collider.transform.selectControl.selected = true;
         }
     }
 }

and in the clicked object, which actually is Tower/Tower (one a child of the other). Both the code and the collider are in the child.

 var selected : boolean;
 private var ring : boolean;
 var selectRing : Transform;
 
 function Select () {
     if (selected) {
         if (!ring) {
             Instantiate(selectRing,Vector3(0,0.001,0),transform.rotation);
             ring = true;
         }
     }
 }