GameObject Instantiation - NullReferenceException

I am trying to instantiate an object in front of the camera whenever the player changes the item they are holding from the inventory. I keep getting the following error:

NullReferenceException: Object reference not set to an instance of an object

(Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.String cacheKeyName, System.Type[] cacheKeyTypes, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetDispatcher (System.Object target, System.Object[] args, System.String cacheKeyName, Boo.Lang.Runtime.DynamicDispatching.DispatcherFactory factory) Boo.Lang.Runtime.RuntimeServices.GetProperty (System.Object target, System.String name) UnityScript.Lang.UnityRuntimeServices.GetProperty (System.Object target, System.String name) InventoryScript.LateUpdate () (at Assets/Scripts/InventoryScript.js:153))

I have modified the script so that it just requires an object to be selected in the editor and keep getting the error. The plan is to use the object ID to point to the correct GameObject in an array of GameObjects.

This is my script, the error occurs on Line 153, the GameObject is set to the prefab of a pan at the moment.

Any ideas? Thanks in advance

 import System.Collections.Generic;
 
 var maincamera : GameObject;
 
 //handles adding of items
 var AddNewItem : boolean = false;
 var AddNewItem_id : int = 0;
 
 //handles item selection from main inventory
 var equip_selection : int = 0;
 var print_size : int;
 var timer : float = 0.1;
 var found_slot_equip : boolean = false;
 var found_slot_inv : boolean = false;
 var slot_to_equip : int;
 var slot_to_inv : int;
 var inventory_visible : boolean = false;
 var equip_visible : boolean = false;
 var input_timer : float = 0.1;
 var update_timer : float = 0.1;
 var equip_view_timer : float = 1;
 var Obj_inst : int;
 var Obj_inst_prefab : GameObject;
 var ActiveObject : GameObject;
 var copy_from_actual : int;
 
 //handles the scrollable print array
 var stagger : int;
 var limit : int;
 var mid_point : int;
 
 //holding all of our items
 var items : Item[];
 var world_pf = new GameObject[16];
 var hold_pf = new GameObject[16];
 var inventory_size : int = 30;
 var inventory_items : int = 0;
 var equip_size : int = 9;
 var equip_items : int = 0;
 
 //changeing of gameobjects
 var change_holding_item : boolean = false;
 
 //inventory
 var MainInventory : List.<Item> = new List.<Item>();
 var EquipMenu : List.<Item> = new List.<Item>();
 
 function Start ()
 {
     mid_point = ((equip_size - 1) / 2);
     stagger = equip_selection - mid_point;
     limit = equip_size - 1;//limit to copy from
     //fill the arrays
     for(var n = 0; n < inventory_size; n++)
     {
         MainInventory.Add(items[0]);
         inventory_items = 0;
     }
     for(var b = 0; b < equip_size; b++)
     {
         EquipMenu.Add(items[0]);
     }
 
     update_print = true;
 
 }
 
 function LateUpdate ()
 {
 
 if( input_timer < 0)
 {
 //---------------TOGGLE INVENTORY SHOW/HIDE------------------
     if(Input.GetButton("Inventory"))
     {
         if( inventory_visible)
         {
             inventory_visible = false;
         }
         else
         {
             inventory_visible = true;
         }
     
     
     }
 
 input_timer = 0.1;
 
 }
 
 
 
 if( update_timer < 0)
 {    
 
     //---------------ADD NEW ITEMS------------------
     if( AddNewItem)
     {
         //find empty slot to place item
         for(var f = -inventory_size; f > 0; f++)
         {
             found_slot_inv = false;
             if(MainInventory[f].Name == "Empty Slot" && !found_slot_inv)
             {
                 slot_to_inv = f;
                 found_slot_inv = true;
             }
          }
          //place item in empty slot
         MainInventory[slot_to_inv] = items[AddNewItem_id];
                 
         AddNewItem = false;    
         AddNewItem_id = 0;//default to empty slot incase
     }
 
     //---------------CHANGE SELECTION WITH MOUSEWHEEL------------------
     if( Input.GetAxis("Mouse ScrollWheel") > 0)
     {
         equip_selection++;    
         equip_view_timer = 1;
         change_holding_item = true;
     }
     if( Input.GetAxis("Mouse ScrollWheel") < 0)
     {
         equip_selection--;
         equip_view_timer = 1;
         change_holding_item = true;
     }
     //---------------HANDLE INVENTORY LIMITS FOR SELECTION------------------
     if( equip_selection > equip_size - 1)
     {
         equip_selection = 0;
     }
     if( equip_selection < 0)
     {
         equip_selection = equip_size - 1;
     }
     
     //---------------EQUIP INVENTORY ONLY VISIBLE FOR A FEW SECONDS------------------
     if( equip_view_timer < 0 && equip_visible)
     {
         equip_visible = false;
     }
     else if( equip_view_timer > 0 && !equip_visible)
     {
         equip_visible = true;
     }
     
     //---------------OBJECT INSTANTIATION WHEN SELECTION CHANGES------------------
     if (change_holding_item)
     {
         Instantiate( Obj_inst_prefab , 
         maincamera.transform.position, 
         Quaternion.Euler( maincamera.rotation.eulerAngles.x,maincamera.rotation.eulerAngles.y,maincamera.rotation.eulerAngles.z));
         change_holding_item = false;
     }
 
 
 
     update_timer = 0.1;
 }    
     
     
     
     
 
     update_timer -= Time.deltaTime;
     input_timer -= Time.deltaTime;
     equip_view_timer -= Time.deltaTime;
 
 }
 
 
 function OnGUI ()
 {
 
 //---------------PRINT THE MAIN INVENTORY + HANDLE OBJECT MANAGEMENT------------------
 if( inventory_visible)
 {        
     print_size = inventory_size;
     var y = 0;//variables for grid structure
     var n = 0;
 
     for(var x = 0; x < print_size; x++)
     {
         if(y > 4){y=0;n++;}
         if( GUI.Button(Rect(Screen.width/2 - 125 + (50 * y), Screen.height/2 - 150 + (50 * n), 50, 50), MainInventory[x].Name ))
         {
             found_slot_equip = false;
             //equip to empty slot
             if( Input.GetButton("lshift"))
             {    
                 for( var g = 0; g < (equip_size - 1); g++)
                 {
                     if( EquipMenu[g].Name == "Empty Slot" && !found_slot_equip)
                     {
                         slot_to_equip = g;
                         found_slot_equip = true;
                     }
                 }
             }
             
             //equip to selected slot
             if( Input.GetButton("lshift") == false)
             {    
 
                 found_slot_equip = true;
 
             }
             //if slot is found then equip the item
             if( found_slot_equip)
             {
                 var item : Item = EquipMenu[slot_to_equip];        
                 //set the new equip item    
                 EquipMenu[slot_to_equip] = MainInventory[x];
                 //set the new inventory item
                 MainInventory[x] = item;
                 //for empty slots transferred update variables
                 if( item.Name == "Empty Slot")
                 {
                 inventory_items--;
                 equip_items++;
                 }
             }        
         }
         y++;
     }
 }
 //---------------PRINT THE EQUIP MENU DEPENDING ON CURRENT SELECTION-------------------
 if( equip_visible)
 {
     for(var v = 0; v < equip_size; v++)
     {
         
         var index = TranslateSelection(v);
             
         if( GUI.Button(Rect(Screen.width - 100, Screen.height/2 - 150 + (50 * v), 100, 50), EquipMenu[index].Name ))
         {    
         
         }
     }
 
 
 
 
 }
 
 
 //---------------PRINT CURRENT ITEM WE ARE USING------------------
 GUI.Label(Rect(Screen.width - 100, Screen.height - 50, 100, 20), "selection" );
 
 
     
             
 }
 
 function TranslateSelection ( iteration : int )
 {
 //---------------TRANSLATE ITERATION TO THE ITEM INDEX FOR PRINTING-----------------
     var translation = equip_selection - mid_point;//iteration + translation = item to print
     var index_toprint = iteration + translation;
     var upperlimit = equip_size - 1;
     var lowerlimit = 0;
     
     if( index_toprint > upperlimit)
     {
         index_toprint -= equip_size;
      
     } 
     if( index_toprint < lowerlimit)
     {
         index_toprint += equip_size;
     }
     
     return index_toprint;
 
 
 
 }