Object reference not set error

Hi,

I have an "Object reference not set to instance of object error in my game and looking at the line of code that it has the problem with I can't figure out what is going wrong, any help or advice would be appreciated.

Here's a screenshot of the game object that I'm having the issue with-

Here is the code that has the error with the error message line of code highlighted -

 using UnityEngine;
 using System.Collections;
 
 public class plateSwitch : MonoBehaviour {
 
     public Sprite plate, setting;
     private SpriteRenderer spriteRenderer;
 
     void Start() {
         spriteRenderer = gameObject.GetComponent<SpriteRenderer>();
         SetVolume(PlayerPrefs.GetInt("volume", 50) != 50 ? false : true);
     }
 
     void Update() {
         if(isTouched()) {
             SetVolume(PlayerPrefs.GetInt("volume", 50) != 50 ? true : false);
         }
     }
 
     private void SetVolume(bool IsOn) {
         if(IsOn) {
             spriteRenderer.sprite = plate;
             PlayerPrefs.SetInt("volume", 50);
         } else {
             spriteRenderer.sprite = setting;
             PlayerPrefs.SetInt("volume", 0);
         }
     }
 
     public bool isTouched() {
         bool result = false;
         if(Input.touchCount == 1) {
             if(Input.touches[0].phase == TouchPhase.Ended) {
                 Vector3 wp = Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position);
                 Vector2 touchPos = new Vector2(wp.x, wp.y);
                 if (GetComponent<Collider2D>() == Physics2D.OverlapPoint(touchPos)) {
                     result = true;
                 }
             }
         }
         if(Input.GetMouseButtonUp(0)) {
             ***Vector3 wp = Camera.main.ScreenToWorldPoint(Input.mousePosition);***
             Vector2 mousePos = new Vector2(wp.x, wp.y);
             if (GetComponent<Collider2D>() == Physics2D.OverlapPoint(mousePos)) {
                 result = true;
             }
         }
         return result;
     }
 
 }

I've been stuck with this, and I just don't know enough sharp myself at the moment to solve the issue.

Vector3 wp = Camera.main.ScreenToWorldPoint(Input.mousePosition);

Is the line in question. Thanks again for any help or advice that can point me in the right direction to fix this.