Focus button with gui.controlfocus

I have i my List "_lootitems" where i add gameobjects to, when i rightclick on the object, which then adds a button to my inventory. But when i try to focus the first button, things goes wrong... Can anyone plz tell me how i can focus a sertain (the first) button when tab is pushed? i want the focus to go the the next button when tab is pressed again.

here is my code:

  using UnityEngine;
     using System.Collections;
     using System.Collections.Generic;
     
     public class Collect_Inventory : MonoBehaviour {
         
         //raycast
         public float Range = 20f;
         
         //Inventory    
         private bool _DisplayInventoryWindow = true;
         private const int Inventory_Window_ID = 1;
         private Rect _InventoryWindowRect = new Rect(10f,10f,300,290f);
         private static int _InventoryRows =4;
         private static int _InventoryCollums = 3;
         private int _InventoryCheck = _InventoryRows * _InventoryCollums;
         private bool InvFull = false;
     
     
         //Store Item - modify Arms and Legs later, into feet/Hands, unter/upper - Arm/Leg
         public List<GameObject> _LootItems = new List<GameObject> ();
         public List<string> ItemNames = new List<string> ();
         public List<bool> ButtonSelected = new List<bool> ();
     
         //Button Prop
         private float ButtonWidth = 40f;
         private float ButtonHeight = 40f;
         private float _offset = 1.1f;
     
     
         // Use this for initialization
         void Start () 
         {
             ButtonSelected = new List<bool> (ItemNames.Count);
         }
         
     
         void Update () 
         {
     
                     //inventory
                     if (Input.GetKeyUp (KeyCode.Alpha1) && _DisplayInventoryWindow == true)
                             _DisplayInventoryWindow = false;
                     else
     
             if (Input.GetKeyUp (KeyCode.Alpha1) && _DisplayInventoryWindow == false)
                             _DisplayInventoryWindow = true;
     
     //        if (Input.GetKeyDown (KeyCode.Tab) && _DisplayInventoryWindow == true) {
     //            int counter = 0;
     //                        for (int i = 0; i < _LootItems.Count; i++) {
     //                                if (GUILayout.Toggle (counter == i, _LootItems [i].name, "Button"))
     //                                        counter = i;
     //                        }
     //                }
     //        if (Input.GetKeyUp (KeyCode.Q) && _DisplayInventoryWindow == true)
     //        _LootItems.Remove(den der er markeret)
                     
                     // raycast
                     if (Input.GetMouseButtonDown (1) && InvFull == true){
                              
                               print ("Inventory is Full!!\n" +
                                     "Drop some Items for more space");
                 return;
             }
             if (Input.GetMouseButtonDown (1) && InvFull==false) {
     
                             
                             Ray ray = new Ray (Camera.main.transform.position + Camera.main.transform.forward, Camera.main.transform.forward);
                             RaycastHit HitInfo;
     
                                                                                     
                 if (Physics.Raycast (ray, out HitInfo, Range) && HitInfo.transform.transform.tag == "Robot") {
                                 GameObject go = HitInfo.collider.transform.parent.gameObject;
                     int nbr4 =0;
                     nbr4++;
                             ItemNames.Add(nbr4.ToString());
                             Destroy (go);
                             _LootItems.Add(go);
                             }
                     }
         }
     
     
         //Loot Window:
         void OnGUI()
         {
     
     
             if(_DisplayInventoryWindow == true)
             _InventoryWindowRect = GUI.Window (Inventory_Window_ID, _InventoryWindowRect, InventoryWindow, "Inventory"); 
             
         }
     
          
          void InventoryWindow (int id)
         {                    
             int nbr = -1, nbr2 = 0, nbr3 = 0;
     
                     foreach (GameObject go in _LootItems) 
         {
                 if(_LootItems.Count != nbr3)
             {
                 nbr++;
                 nbr3++;
                 
     
                                     if (nbr == _InventoryCollums) {
                                             nbr -= _InventoryCollums;
                                             nbr2++;
                                     }
                     if (_LootItems.Count >= 0)
                     {
                         for(int i = 0; i < _LootItems.Count; i++) 
                         {
                         GUI.SetNextControlName(ItemNames[i]);
                         GUI.Button (new Rect (_offset * (nbr * ButtonWidth), 20 + _offset * (nbr2 * ButtonHeight), ButtonWidth, ButtonHeight), nbr3.ToString());
     
                                 if (Input.GetButton("tab")) 
                             {
                                 GUI.FocusControl(nbr3.ToString());
     
                             }
     
                         }
                     }
     
             }
                                         
                         if(nbr3 == _InventoryCheck)
                         {
                         InvFull = true;
                         }
     
         }
                             
     
                             for (int y = 0; y < _InventoryRows; y++)
                                     for (int x = 0; x < _InventoryCollums; x++)
                                             GUI.Label (new Rect (_offset * (x * ButtonWidth), 20 + _offset * (y * ButtonHeight), ButtonWidth, ButtonHeight), "", "box");
     
             return;
         }
     }