Destroying Instantiated Prefab

Hello, I am working on an inventory system but I have got a bit of an issue with destroying an instantiated prefab from the scene by clicking a GUI.Button. In my code I have made the item spawn and become a child of the player's hand so it follows the position. But when I click a different button which is supposed to remove it from the scene I am having issues with getting this to work. I've tried Destroy(handItem.gamObject); but I get an error message saying "NullReferenceException: Object reference not set to an instance of an object" I don't know how to do this and any help would be great! Here's my code for the inventory, line 92 is where the button is which I want to press to delete the instance of the prefab. And line 61 is where the prefab is being instantiated.

 import System.Collections.Generic;
 
 var items : Item[];
 
 var currentlyEquiped : Item[];
 //[0 = handItem, 1 = headItem, 2 = shoulder/neckItem, 3 = torsoItem]
 
 var mainInventory : List.<Item> = new List.<Item>();
 
 private var inventoryColumns : int = 3;
 private var inventoryRows : int = 5;
 
 var inventoryOpen : boolean = false;
 var currentState : String = "Open Inventory";
 
 var equipMenuOpen : boolean = false;
 var currentEquipText : String = "Open Equip Menu";
 
 var buttonWidth : int = 50;
 var buttonHeight : int = 50;
 
 var handPos : Transform;
 
 function Update(){
     if(Input.GetKeyDown(KeyCode.I)){
         inventoryOpen = !inventoryOpen;
     }
     if(inventoryOpen){
         currentState = "Close Inventory";    
     }
     if(!inventoryOpen){
         currentState = "Open Inventory";
     }
     if(equipMenuOpen){
         currentEquipText = "Close Equip Menu";
     }
     if(!equipMenuOpen){
         currentEquipText = "Open Equip Menu";
     }
 }
 
 function OnGUI(){
 if(GUI.Button(Rect(10,Screen.height-40,120,25),currentState)){
     inventoryOpen = !inventoryOpen;
 }
 
 if(inventoryOpen){
 GUI.Box(Rect(10,Screen.height-375,350,325),"Inventory");
 
 var i : int = 0;
     for(var y = 0; y < inventoryRows; y ++){
         for(var x = 0; x < inventoryColumns; x ++){
             
             if(i < mainInventory.Count){
                 if(GUI.Button(Rect(23+(52*x),Screen.height-340+(52*y),50,50),
                 GUIContent(mainInventory[i].icon,"Name : " + mainInventory[i].name + "\n" + "Description : " + 
                 mainInventory[i].desc + "\n" + "Rarity : " + mainInventory[i].rarity.ToString()))){
 /*************Check item type*************/
                         if(mainInventory[i].itemType == mainInventory[i].itemType.handItem){
                             currentlyEquiped[0] = mainInventory[i];
                             handItem = Instantiate(currentlyEquiped[0].physicalItem, handPos.position, handPos.rotation);
                             handItem.parent = handPos;
                             //Add rotation    
                         }
                     
                     mainInventory.RemoveAt(i);
                 }    
             }else{
                 GUI.Box(Rect(23+(52*x),Screen.height-340+(52*y),50,50),"");
             }
             i++;
         }
 
     }
     //Display the ToolTip
 GUI.Box(Rect(186,Screen.height-340,160,258),"Item Information");
 GUI.Label(Rect(195,Screen.height-320,145,200), GUI.tooltip);
 
 if(GUI.Button(Rect(340,Screen.height-375,20,20),"X")){
     inventoryOpen = !inventoryOpen;
 }
 
 if(GUI.Button(Rect(10,Screen.height-375,120,20),currentEquipText)){
     equipMenuOpen = !equipMenuOpen;
 }
 
 if(equipMenuOpen){
     GUI.Box(Rect(361,Screen.height-375,180,180),"Equip Menu");
 
 //Equipped Items buttons
 if(currentlyEquiped[0] != null){
     if(GUI.Button(Rect(370,Screen.height-340,30,30),currentlyEquiped[0].icon)){
         mainInventory.Add(currentlyEquiped[0]);
         currentlyEquiped[0] = null;
         Destroy(handItem.gameObject);
     }
 }
 
 }
 
 
 }
 
 
 }
 
 function AddItem(Name : String){
     if(Name == "CupOfCoffee"){
         mainInventory.Add(items[0]);
     }
     if(Name == "Popsicle"){
         mainInventory.Add(items[1]);
     }
     if(Name == "BasicSword"){
         mainInventory.Add(items[2]);
     }
 }