nullreferenceexception object reference not set to an instance of an object

I´m realy new at unity and javascript so i need some help. I´m trying to make a gun aim and i get this null error. Here is my script:

 var gun : Transform;
 var nextPos = 0.0;
 var nextField = 40.0;
 var nextPos2 = -0.2;
 var dampVelocity = 0.4;
 var dampVelocity2 = 0.4;
 var dampVelocity3 = 0.4;
 
 function Update () {
    var newPos = Mathf.SmoothDamp(gun.transform.localPosition.x, nextPos, dampVelocity, .3);
    var newField = Mathf.SmoothDamp(Camera.main.fieldOfView, nextField, dampVelocity2, .3);
    var newPos2 = Mathf.SmoothDamp(gun.transform.localPosition.y, nextPos2, dampVelocity3, .3);
 
    gun.transform.localPosition.x = newPos;
    gun.transform.localPosition.y = newPos2;
    Camera.main.fieldOfView = newField;
 
    if (Input.GetButton("Fire2")) {
        //adjust viewpoint and gun position
        nextField = 40.0;
        nextPos = 0.0;
        nextPos2 = -0.2;
 
        //slow down turning and movement speed
        GetComponent("FPSWalker").speed = 1.5;
        GetComponent("MouseLook").sensitivityX = 2;
        camera.main.GetComponent("MouseLook").sensitivityX = 2;
        camera.main.GetComponent("MouseLook").sensitivityY = 2;
    } else {
        //adjust viewpoint and gun position
        nextField = 60.0;
        nextPos = 0.5;
        nextPos2 = -0.4;
 
        //speed up turning and movement speed
        GetComponent("FPSWalker").speed = 6;
        GetComponent("MouseLook").sensitivityX = 6;
        camera.main.GetComponent("MouseLook").sensitivityX = 6;
        camera.main.GetComponent("MouseLook").sensitivityY = 6;
    }
 }

Did you remember to assign the gun transformation in the editor?
I don’t see the gun being assigned in the code.

Hi thx for replying so fast. I’m not sure if i did that. How do i do it?