Trouble setting up an inventory system to interact with game objects.

Hello, i'm trying to implement an invnetory system but having some issues. i made a script basedon some tutorials but am having some trouble when it comes to interact with a game object and making it add something to the inventory. Probably not using the best solutions because i'm not very good at programing. When i run the scripts there is no errors, but when i try to add the item it gives an error :

NullReferenceException: Object reference not set to an instance of an object Tree01.tree01Window (Int32 windowID) (at Assets/Scripts/Tree01.js:36) UnityEngine.GUI.CallWindowDelegate (UnityEngine.WindowFunction func, Int32 id, UnityEngine.GUISkin _skin, Int32 forceRect, Single width, Single height, UnityEngine.GUIStyle style) (at C:/BuildAgent/work/d3d49558e4d408f4/artifacts/EditorGenerated/GUI.cs:1395)

Here are the scripts i'm using:

Object script.

 #pragma strict
 var x = 0.0;
 var y = 0.0;
 
 private var wop : boolean = false;
 private var gcc : boolean = false;
 private var dgo : boolean = false;
 
 function Start () {
 
 }
 
 function OnMouseOver(){
     if (Input.GetMouseButtonDown(1) && wop == false){
         wop = true;
     }
 }
 
 function OnGUI(){
     var mousePos = Vector3(Input.mousePosition.x, -(Input.mousePosition.y-Screen.height),0);
     var windowRect : Rect = Rect(x,y,150,80);
     if(Input.GetMouseButtonDown(1)){
         x = mousePos.x;
         y = mousePos.y;
     }
     if (wop==true){
         windowRect = GUI.Window (1, windowRect, tree01Window, "A tree, what now?");
     }
 }
 
 function tree01Window (windowID : int){
     
     if (GUI.Button(Rect(15,20,120,20), "Gather Wood")){
         print("Gathering wood...");
         var player = GameObject.Find("Player");
         player.GetComponent(CollectResources).GetWood();
         
     }
     if (GUI.Button(Rect(15,50,120,20), "Let it be for now")){
         print("Leaving it...");
         gcc = true;
         closeWindow();        
     }
 }
 
 function closeWindow(){
     if (gcc == true){
         wop = false;
     }
 }
 
 
 
 function Update () {
 
 }

Collect Resources script:

 #pragma strict
 import System.Collections.Generic;
 
 var collectWood : ItemClass [];
 var collectStone : ItemClass [];
 var Inventory : Inventory;
 
 function Start () {
 
     Inventory = GetComponent ("Inventory") as Inventory;
 
 }
 
 function OnGUI () {
         
 }
 
 function GetWood (){
     
     for (var x = 0; x < collectWood.Length; x++){
         Inventory.playerInventory.Add(collectWood[x]);
     }
 }
 
 function GetStone (){
     
     for (var x = 0; x < collectStone.Length; x++){
         Inventory.playerInventory.Add(collectStone[x]);
     }
 }

Inventory:

 #pragma strict
 
 import System.Collections.Generic;
 
 var playerInventory : List.<ItemClass> = new List.<ItemClass>();
 var scrollView : Vector2;
 var invSkin : GUISkin;
 var invOpen : boolean = false;
 
 function Update () {
 
     if (Input.GetKeyDown("i")){
         invOpen =! invOpen;
     }
 }
 
 function OnGUI () {
 
     var windowRect: Rect = Rect(Screen.width-400,0,400,200);
     if(invOpen == true){    
         windowRect = GUI.Window(0, windowRect, InvWindow, "Inventory");
         return;
     }
 }
 
 function InvWindow (windowID: int) {
     GUI.skin = invSkin;
     scrollView = GUILayout.BeginScrollView(scrollView, GUILayout.Width(380), GUILayout.Height(180));
 //    if (GUI.Button(Rect(250,0,10,10), GUIContent (" ", "Close"))){
 //        invOpen = false;
 //    }
 //    GUI.Label(Rect(250,20,100,40), GUI.tooltip); // displays tooltips on mouse over passed by GUIContent    
     for (var x = 0; x < playerInventory.Count; x++){
         GUILayout.BeginHorizontal();
         if(GUILayout.Button (playerInventory[x].icon, GUILayout.Width(25), GUILayout.Height(25)))
             {
             playerInventory.RemoveAt(x);
             return;
             }
         GUILayout.Box (playerInventory[x].name, GUILayout.Width(80), GUILayout.Height(25));
         GUILayout.Box (playerInventory[x].description, GUILayout.Width(190), GUILayout.Height(25)); 
         GUILayout.Box (playerInventory[x].weight, GUILayout.Width(20), GUILayout.Height(25));
         GUILayout.Box (playerInventory[x].ammount, GUILayout.Width(20), GUILayout.Height(25));        
         GUILayout.EndHorizontal();            
     }
     GUILayout.EndScrollView();
 }
 

Any ideas on how to work around this? Thanks in advance.