Continuous Pointer with Physics RayCasting within AR Scene

I'm trying to created a continuous raycast from the center of the viewport that triggers a script on the other object it hits (to activate UI and a few other functions. The idea is that there are lots of objects in the AR scene and the user reveals UI/information about them by pointing their camera towards it. I'm essentially trying to achieve this effect but within an AR scene:

I have the script working on a Desktop Build with an FPS Controller and have it working with AR Foundation if the raycast is triggered by touching the screen, but anytime I try to have it work without input from the user other than moving it around I get this error in the debug:
NullReferenceException: Object reference not set to an instance of an object.

Here's my code (color Change is the script on the object that activates the UI and other "selected" functions):

Code (CSharp):

1. using System.Collections;
2. using System.Collections.Generic;
3. using UnityEngine;
4.  
5. public class raycastSelectionAR : MonoBehaviour
6. {   [SerializeField] private Color highlightMaterial;
7.     [SerializeField] private Color defaultMaterial;
8.     [SerializeField] private string selectableTag= "Subjects";
9.     [SerializeField] private Camera cam;
10.     //public GameObject UIElements;
11.     void Awake(){
12.         cam = GetComponent<Camera>();
13.     }
14.     void Update()
15.     {
16.         var ray =cam.ViewportPointToRay(new Vector3(0.5f,0.5f,0));
17.         RaycastHit hit;
18.         if(Physics.Raycast(ray, out hit))
19.         {
20.             var selection = hit.transform;
21.             if (selection.CompareTag(selectableTag))
22.             {  
23.                 Debug.Log("cast has hit a profile graphic!");
24.                 Debug.Log(hit.collider.gameObject.name);
25.              
26.                 selection.gameObject.GetComponent<colorChange>().hitByRay = true;
27.                
28.             }
29.         }
30.     }
31. }

Any help greatly appreciated!

 

—> In Editor Testing
// debug Ray
Debug.DrawRay(ray.origin, ray.direction * rayLength, Color.red);

—>In Game View used line renderer

 

Add Condition:

If(hit.collider != null)
Debug.Log(hit.collider.gameobject.name);