SpringJoint2D Cannot be referenced properly

So I was trying to make my joint, and when the player stops holding with his mouse, a projectile would be fired from the sling. and i get this problem when releasing the projectile from the sling :

 NullReferenceException: Object reference not set to an instance of an object
 SlingBackController.ShootProjectile ()

as if i haven't referenced anything for my breakforce to do but i did from this script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class SlingBackController : MonoBehaviour
 {
     public float MaxDistance = 5f;
     public Transform MiddlePosition; 
     private static SlingBackController _instance;
     private SpringJoint2D CurrentSpringJoint;
 
     public static SlingBackController Instance
     {
         get
         {
             if (_instance == null)
             {
                 var obj = GameObject.FindGameObjectWithTag("SlingBack");
                 if (obj != null)
                 {
                     _instance = obj.GetComponent<SlingBackController>();
                 }
             }
             return _instance;
         }
         set
         {
             _instance = value;
         }
     }
 
 
     // Start is called before the first frame update
     void Awake()
     {
         Instance = this;  
     }
 
     void start()
     {
         CurrentSpringJoint = GetComponent<SpringJoint2D>();
     }
 
     // Update is called once per frame
     void Update()
     {
         
     }
 
     public void ShootProjectile()
     {
         CurrentSpringJoint.breakForce = 1f;
         CurrentSpringJoint = null;
     }
 }
 

the spring was clearly referenced and shown as a SpringJoint2D, and it still doesn't break when player releases the projectile through my script, the problem is pointing to the ShootProjectile() problem, Can anyone please help?