Null Reference Exception when changing material shader

I am trying to create a script that makes all the materials of all objects obstructing the view of the player become transparent, and become opaque when they are no longer obstructing the view. It seems to work fine in the editor now, but whenever I go from having >= 1 object blocking the view to having no objects blocking the view I am flooded with null reference exception errors that point specifically to mats = rends.materials; near the bottom. Through debugging i've found that the error seems to stem from specifically referencing the materials of gameobjects in the if (hits.length == 0) { statement. Any thoughts?

using UnityEngine; using System.Collections;

public class ObstructionFade : MonoBehaviour {

 public Camera cam;
 public Renderer[] rend;
 public Renderer[] lastRend;
 public RaycastHit[] hits;
 public Material[] mats;
 public int i;
 LayerMask mask;

 void Start () {
     mask = LayerMask.GetMask ("Default");
 }

 // Update is called once per frame
 void LateUpdate () {



     i = 0;
     Ray camRay = cam.ScreenPointToRay (cam.WorldToScreenPoint (transform.position + new Vector3 (0, 2, 0)));
     Debug.DrawLine (cam.transform.position, transform.position);




     //gets all things the raycast hits
     hits = Physics.RaycastAll (camRay, Vector3.Distance (Camera.main.transform.position, transform.position) - 3, mask);




     //for every thing the raycast hits
     foreach (RaycastHit hit in hits){

         //set last renderer hit to the current renderer
         lastRend[i] = rend[i];

         //get the current renderer hit and make it transparent
         rend[i] = hit.collider.gameObject.GetComponent<MeshRenderer> ();
         mats = rend [i].materials;
         foreach (Material mat in mats) {
             mat.color = new Color (mat.color.r, mat.color.g, mat.color.b, 0.3f);
             mat.shader = Shader.Find ("Legacy Shaders/Transparent/Diffuse");
         }

         //if the last and current renderer dont match make the last renderer opaque
         if (lastRend[i] != rend[i]){
             mats = lastRend [i].materials;
             foreach (Material mat in mats) {
                 mat.color = new Color (mat.color.r, mat.color.g, mat.color.b, 1.0f);
                 mat.shader = Shader.Find ("Standard");
             }

         }
         //increment i
         i++;
     }



     //if the raycast hits nothing make all previously hit objects opaque
     if (hits.Length == 0) {
         foreach (Renderer rends in rend) {
             mats = rends.materials;
             foreach (Material mat in mats) {
                 mat.color = new Color (mat.color.r, mat.color.g, mat.color.b, 1.0f);
                 mat.shader = Shader.Find ("Standard");
             }
         }
     }
 }

}