How to object pool an Explosion

Hey soo i have 3 scripts, an object pooler script, a scipt that spawns a bomb at random location and a sticky bomb script. I want to make the explosion occur 5 seconds after spawning each the bomb prefab, however it only occurs once with the first prefab and the rest of the bombs do not explode. The error that comes up is: NullReferenceException: Object reference not set to an instance of an object StickyBomb.Explode () (at Assets/Worpman/Scripts/StickyBomb.cs:22)

Things to note: I have pooled both the sticky bomb and explosion prefab(the detonation prefab from Asset Store).| This is for a mobile game hence the reason why im doing this.| Sticky bomb is attach to the bomb prefab.| Spawn locations are just empty gameobjects at diffrent locations on the terrain.| A pooler gameobject has both the spawn script and object pooler script

STICKY BOMB SCRIPT;

 using UnityEngine;
 using System.Collections;
 
 public class StickyBomb : MonoBehaviour {
   //  public GameObject explosionPrefab;
 
 /*    void OnCollisionEnter(Collision col)
     {
         this.GetComponent<Rigidbody>().isKinematic = true;
         Invoke("Explode", 5);
     } */
 
         void Update()
     {
         Invoke("Explode", 5);
     }
     
     void Explode()
     {
        GameObject explosionPrefab = ObjectPooler.current.GetPooledObject("explosion");
       //  Instantiate(explosionPrefab, this.transform.position, this.transform.rotation);
          explosionPrefab.transform.position = this.transform.position;
          explosionPrefab.transform.rotation = this.transform.rotation;
          explosionPrefab.SetActive(true);
      
          gameObject.SetActive(false);
       
     }
 }

SCRIPT THAT SPAWNS THE BOMB

 using UnityEngine;
     using System.Collections;
     
     public class SpawnBomb : MonoBehaviour {
     
         public GameObject[] Spawner;
         Transform bombpos;
         public int SP_amount;
         public float spawn_Wait;     // Always changing
         public float spawn_MostWait;  // longest it will wait for
         public float spawn_LeastWait;  // least it would wait for
         public int start_Wait;         //time to first bomb being created
         int randomPos;
         public bool stop;
     
         // Use this for initialization
         void Start ()
         {
             StartCoroutine(waitBombSpawner());
         }
         
         // Update is called once per frame
         void Update () {
        
             spawn_Wait = Random.Range(spawn_LeastWait, spawn_MostWait); //minimum and maximum
         }
     
         void Awake()
         {
                 
             //Spawner[randomPos].position = bombpos;
          
         }
     
         IEnumerator waitBombSpawner()
         {
             yield return new WaitForSeconds(start_Wait);
             while (!stop)
             {
                 randomPos = Random.Range(0, 6);                        // since its an array it picks a number between 0 and 5
                 GameObject BOMB = ObjectPooler.current.GetPooledObject("bomb");  // bomb from pooler
                 BOMB.transform.position = Spawner[randomPos].transform.position;  //Picks a random location to be spawned
                 BOMB.SetActive(true);                                  //put it back to active
                 yield return new WaitForSeconds(spawn_Wait);
             }     
            
         }
     }

OBJECT POOLER

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class ObjectPooler : MonoBehaviour {
 
     public static ObjectPooler current;
     
 
     [System.Serializable]
     public class ObjectPoolItem
     {
         public GameObject pooledObject;
         public int pooledAmount;
         public bool WillGrow = true;
     }
 
     List<GameObject> pooledObjects;
     public List<ObjectPoolItem> itemsToPool; // Newly added
     // GameObject obj = ObjectPooler.current.GetPooledObject(); use it in another script to call this one
     void Awake()
     {
         current = this;
 
     }
     void Start()
     {
         pooledObjects = new List<GameObject>();
         foreach (ObjectPoolItem item in itemsToPool)
         {  
             for (int i = 0; i < item.pooledAmount; i++)
             {
                 GameObject obj = (GameObject)Instantiate(item.pooledObject);
                 obj.SetActive(false);
                 pooledObjects.Add(obj);
             }
         }
     }
   //  void Update() { }
     public GameObject GetPooledObject(string tag)
     {
         for (int i = 0; i<pooledObjects.Count; i++)
         {
             if (!pooledObjects[i].activeInHierarchy && pooledObjects[i].tag == tag)
             {
                 return pooledObjects[i];
             }
         }
         foreach (ObjectPoolItem item in itemsToPool)
         {
             if (item.pooledObject.tag == tag)
             {
                 if (item.WillGrow)                                                      //Expands the pool
                 {
                     GameObject obj = (GameObject)Instantiate(item.pooledObject);
                     obj.SetActive(false);
                     pooledObjects.Add(obj);
                     return obj;
                 }
               
             }
         }
         return null;
     }
       
 }