Camera NullReferenceException

I have the following script attached to my camera.

 using UnityEngine;
 using System.Collections;
 
 [ExecuteInEditMode]
 public class ScaleWidthCamera : MonoBehaviour 
 {
     public float targetWidth = 1920;
     public float FOV = 42f;
 
     void OnGUI()
     {
         float height = (targetWidth / Screen.width) * Screen.height;
 
         camera.orthographicSize = (height / FOV) / 2f;
     }
 }
 

Sometimes when I'm in the editor I get a NullreferenceException error on the camera. Along with !CompareApproximately (det, 1.0F, .005F)

This makes absolutely no sense since camera clearly exists and I only get this error once. When I run the game this script works just fine (It is used by a slider that adjusts the FOV value).

Before you say anything about using OnGUI(), I did that because if the FOV is changed during Update() or FixedUpdate(), it causes a stuttering movement as the FOV is changed.