Unity3D texture2d error help

please help this error shoes up every time: "DrawGUITexture: texture is null UnityEngine.Graphics:DrawTexture(Rect, Texture) Inventory:DrawInventory() (at Assets/Scripts/Inventory.cs:55) Inventory:OnGUI() (at Assets/Scripts/Inventory.cs:38) " for this script: using UnityEngine; using System.Collections; using System.Collections.Generic;

 public class Inventory : MonoBehaviour {
     public int slotsX, slotsY;
     public GUISkin skin;
     public List<Item> inventory = new List<Item>();
     public List<Item> slots = new List<Item>();
     private bool showInventory;
     private ItemDatabase database;
     
     void Start () {
         
         for(int i = 0; i < (slotsX * slotsY); i++)
         {
             slots.Add(new Item());
             inventory.Add (new Item());
         }
         database = GameObject.FindGameObjectWithTag("ItemDatabase").GetComponent<ItemDatabase>();
         inventory[0] = database.items[0];
         inventory[1] = database.items[1];
     }
     
     void Update()
     {
         if(Input.GetButtonDown("Inventory"))
         {
             showInventory = !showInventory;
         }
     }
     
     void OnGUI () 
     {
         GUI.skin = skin;
         if (showInventory)
         {
             DrawInventory();
         }
     }
     
     void DrawInventory()
     {
         int i = 0;
         for (int x = 0; x < slotsX; x++)
         {
             for (int y = 0; y < slotsY; y++)
             {
                 Rect slotRect = new Rect(x * 60, y * 60, 50, 50);
                 GUI.Box(slotRect, "", skin.GetStyle("Slot"));
                 slots[i] = inventory[i];
                 
                 if(slots[i].itemName != null)
                 {
                      Graphics.DrawTexture(slotRect, slots[i].itemIcon);
                 }
                 
                 i++;
             }
         }
     }
 }