Object reference not set to instance of an object?

I'm not sure why, but I keep on getting an error regarding to NullReferenceException in my script for a game that I'm working on. It keeps on referring to line 96 (or //KeyDisplay.sprite = Resources.Load ("pixel_key") as Sprite;) I try to display a sprite on my UI whenever I walk to pick up a key. On that collision it should be updating the sprite on the UI after colliding with the key, but it just throws that exception out at me. Please help!

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 
 public class Move : MonoBehaviour
 {
     private SpriteRenderer spriteRenderer;
     public Sprite Character_Back, Character_Front, Character_Left, Character_Right, pixel_key;
     public float speed = 1.5f;
     public Vector2 tempSpeed;
     private int count;
     public Text amtOfKeys;
     public Image KeyDisplay;
 
     Animator animator;
 
     void Start ()
     {
         animator = GetComponent<Animator>();
         spriteRenderer = GetComponent<SpriteRenderer>();
         if (spriteRenderer.sprite == null)
             spriteRenderer.sprite = Character_Front;
         count = 0;
         amtOfKeys.text = "";
     }
 
     void Update ()
     {
         
         animator.SetBool("Up", false);
         animator.SetBool("Down", false);
         animator.SetBool("Left", false);
         animator.SetBool("Right", false);
 
         if (Input.GetKey (KeyCode.A)) 
         {
             transform.position += Vector3.left * speed * Time.deltaTime;
             animator.SetFloat ("X-Speed", -speed);
             tempSpeed.x = -speed;
         } 
         else if (Input.GetKey (KeyCode.D)) 
         {
             transform.position += Vector3.right * speed * Time.deltaTime;
             animator.SetFloat ("X-Speed", speed);
             tempSpeed.x = speed;
         }
 
         if (Input.GetKey (KeyCode.W))
         {
             transform.position += Vector3.up * speed * Time.deltaTime;
             animator.SetFloat ("Y-Speed", speed);
             tempSpeed.y = speed;
         }
         else if (Input.GetKey (KeyCode.S))
         {
             transform.position += Vector3.down * speed * Time.deltaTime;
             animator.SetFloat ("Y-Speed", -speed);
             tempSpeed.y = -speed;
         }
 
         if (tempSpeed.y > 0) 
         {
             animator.SetBool ("Up", true);
         } 
         else if (tempSpeed.y < 0) 
         {
             animator.SetBool ("Down", true);
         } 
         else if (tempSpeed.x > 0) 
         {
             animator.SetBool ("Right", true);
         } 
         else if (tempSpeed.x < 0) 
         {
             animator.SetBool ("Left", true);
         } 
         else 
         {
         }
     }
 
     void OnTriggerEnter2D(Collider2D other) 
     {
         if (other.gameObject.CompareTag("Key"))
         {
             other.gameObject.SetActive(false);
             count++;
             SetAmtOfKeys ();
         }
     }
 
     void SetAmtOfKeys()
     {
         amtOfKeys.text = "\tKeys : " + count.ToString ();
         //KeyDisplay.sprite = Resources.Load<Sprite> ("pixel_key") as Sprite;
     }
 }