How to Fix "NullReferenceException"? Aim Down Sights Script

Hi community, I am kind of new to the whole Unity/progamming thing and am working on a school assignment.

With pretty much every script I've done, I have come across the error: "NullReferenceException", and, in most cases, have been quite successful in fixing the issue.

But, in the case of this AimDownSights Script, I have been less successful. I received the following error:

 NullReferenceException: Object reference not set to an instance of an object
 AimDownSights.Update () (at Assets/AimDownSights.js:44)

My script is as follows:

   var gun : Transform; 
   var curField = 60.0;
   private var dampVelocity2 = 0.4;
   var nextVector : Vector3; //Sights Transform
   var curVector : Vector3; //Normal Transform
   
   var curY = 234.8665; //Normal Rotation of Gun
   var nextY = 230.7; //Rotation of Gun when looking down Sights
  
   class weaponStats {
       var curDamage : int; //Normal Weapon Damage
       var nextDamage : int; //Sights Weapon Damage
       var gunScript : String;
       var mainCam : Crosshair_Script; 
   }

   var speed : float = 0.1;
   var stats : weaponStats;
   
   function Update () {
       //Smooth Changine Vield of View of Camera
      var newField = Mathf.SmoothDamp(Camera.main.fieldOfView, curField, dampVelocity2, .3);
      Camera.main.fieldOfView = newField;
   
      if (Input.GetButton("Fire2")) { 
          //adjust viewpoint and gun position
          curField = 40;
          //Rotating Gun to look down Camera Perfectly
          transform.localRotation.eulerAngles = Vector3(0,nextY,0);
   
          //slow down turning and movement speed
          transform.root.GetComponent(CharacterMotor).movement.maxForwardSpeed = 2.5;
          //Adjust Gun Damage
          GetComponent(stats.gunScript).damage = stats.nextDamage;
          stats.mainCam.enabled = false;
          //Moving the gun
          transform.localPosition = Vector3.Lerp(transform.localPosition,nextVector, 0.3);
      } else {
          //adjust viewpoint and gun position
          curField = 60.0;
          transform.localRotation.eulerAngles = Vector3(0,curY,0);
          //speed up turning and movement speed
          transform.root.GetComponent(CharacterMotor).movement.maxForwardSpeed = 10;
          stats.mainCam.enabled = true;
          GetComponent(stats.gunScript).damage = stats.curDamage;
          transform.localPosition = Vector3.Lerp(transform.localPosition,curVector, speed);
      }
   }

So, I get that this means that the error is on line 44 and it probably has something to do with the "mainCam" but I am not sure how I would go about fixing this. Any help for this particular problem would be appreciated, and some information on how to solve this sort of problem in the future would probably make my life a lot easier.