Unable to instantiate gameobject after using Destroy()

Hello,

I have a script which allows me to create 3 balls: red, blue and green. These orbs spawn at the available spawn positions : pos1, pos2 and pos3. They follow the player, hovering over the player's head.

The goal is to have a certain combination of balls and then press a button to create a power up. When I press space and get the power up, I want to destroy the existing balls, create a spell and start over again ( be able to create more balls) However, after I destroy the balls, I am unable to create them again. I am getting a MissingReferenceException. The problem happens when i try to use Destroy() in the Invoke function towards the very end of the script. I have no idea at which part of the code the problem is occurring exactly!

Here is the script im using.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System;
 
 public class orbController : MonoBehaviour
 {
     int currentOrb = 0;
     int maxOrbs = 3;
 
     public Transform[] spawnPos;
 
     public GameObject airOrb;
     public GameObject fireOrb;
     public GameObject waterOrb;
 
     //public bool allowOrbit = false; // if set to true then orbiting functionality needs to be added
     public bool overRideCurrentOrb = false; // if set to true then "currentOrb" will not be used
 
     public List<GameObject> orbs = new List<GameObject>();
 
     void Start()
     {
         if (maxOrbs < spawnPos.Length)
         {
             maxOrbs = spawnPos.Length;
         }
     }
 
     void Update()
     {
         if (Input.GetKeyDown ("q") && currentOrb < maxOrbs) 
         {
             spawnAirOrb ();
 
         }
 
         if (Input.GetKeyDown ("w") && currentOrb < maxOrbs) 
         {
             spawnFireOrb ();
         }
 
         if (Input.GetKeyDown ("e") && currentOrb < maxOrbs) 
         {
             spawnWaterOrb ();
         }
         InvokeSpell ();
     }
 
     void spawnAirOrb()
     { 
         spawnOrb(airOrb,0);
     }
 
     void spawnFireOrb()
     {
         spawnOrb(fireOrb,1);
     }
 
     void spawnWaterOrb()
     {
         spawnOrb(waterOrb,2);
     }
 
     void spawnOrb(GameObject orb, int index)
     { 
         if (orb != null)
         {
             if (!overRideCurrentOrb)
             {
                 index = currentOrb;
             }
 
             GameObject oldOrb = GameObject.Find(orb.name + "(Clone)");
             Vector3 position = spawnPos [index].position;
             Quaternion rotation = spawnPos [index].rotation;
 
             if (orbs.Count == maxOrbs)
             {
                 for (int i = 0; i < orbs.Count; i++)
                 {
                     if (( (orbs[i].transform.position == position)) ||
                         ((orbs[i] != null)))
                     {
                         oldOrb = orbs[i];
                         orbs.RemoveAt(i);
                         position = oldOrb.transform.position;
                         rotation = oldOrb.transform.rotation;
                         break;
                     }
                 }
             }
 
 
             if (oldOrb != null)
             {
                 bool destroyOrb = (oldOrb.transform.position == position);
 
                     
 
                 if (destroyOrb)
                 {
                     Destroy(oldOrb);
                 }
             }
             GameObject spawnedOrb = (Instantiate (orb, position, rotation) as GameObject);
             spawnedOrb.transform.parent = spawnPos [index].transform;
 
             orbs.Add(spawnedOrb);
 
             currentOrb++;
             if (currentOrb >= maxOrbs)
             {
                 currentOrb = 0;
             }
         }
 
 
 
     }
 
     void InvokeSpell()
     {
         GameObject[] orb1 = GameObject.FindGameObjectsWithTag ("airOrb");
         GameObject[] orb2 = GameObject.FindGameObjectsWithTag ("fireOrb");
         GameObject[] orb3 = GameObject.FindGameObjectsWithTag ("waterOrb");
 
         if(Input.GetKeyDown ("space"))
         {
             if(orb1.Length==3)
             {
                 Debug.Log("3 air orbs bruh");
                 //GameObject spell = (Instantiate (ball, player.position, player.rotation) as GameObject);
                 for (int i = 0; i <= orb1.Length; i++) {
                     Destroy (orb1 [i].gameObject);
                     currentOrb = 0;
 
                 }
 
             }
             else if(orb2.Length==3)
             {
                 Debug.Log("3 fire orbs bruh");
             }
             else if(orb3.Length==3)
                 Debug.Log("3 water orbs bruh");
         }
 
     }
 }