Object reference not set to an instance of an object [HELP!]

I'm getting this error but I have no clue whats wrong with my code. I am not getting any compile errors, but errors when I start the game. The error is:

NullReferenceException: Object reference not set to an instance of an object PlayerCamera.CameraMovement () (at Assets/Scripts/PlayerCamera.cs:185) PlayerCamera.LateUpdate () (at Assets/Scripts/PlayerCamera.cs:141)

Here is the code that gives me the error:

 public void CameraMovement()
     {
         if (playerController.aim) 
         {
             Camera.main.fieldOfView = Mathf.Lerp (Camera.main.fieldOfView, zoomFOV, _deltaTime * lerpSpeed);
 
             _camDir = (aimDirection.x * target.forward) + (aimDirection.z * target.right);
             _targetHeight = normalAimHeight;
             _targetDistance = normalAimDistance;
         } 
         else 
         {
             Camera.main.fieldOfView = Mathf.Lerp (Camera.main.fieldOfView, normalFOV, _deltaTime * lerpSpeed); //Error on this line
 
             _camDir = (normalDirection.x * target.forward) + (normalDirection.z * target.right);
             _targetHeight = normalHeight;
             _targetDistance = normalDistance;
         }