Why am I geting null reference exception while reading xml file?

hi, here is my problem: I'm trying to load information of my game map that are saved in a xml file, but while reading it, it gives me a null reference exception..

here is the code i'm using:

 using UnityEngine;
 using System.Collections;
 using System.IO;
 using System.Xml;
 using System.Xml.Serialization;
 public static class XMLUtility
 {
     static string path = "C:/Users/Dan/Documents/projet beta/Assets/map-xml.xml";
     
 
    
     public static void LoadXML() {
              var xmlSerializer = new XmlSerializer(typeof(itemsDirectory ));
              var stream = File.Open(path, FileMode.Open);
              Debug.Log(stream);
              var deserializedtile = xmlSerializer.Deserialize(stream) as itemsDirectory ;
              
              stream.Close();
              for(int i = 0; i < deserializedtile.tile.Length; i++) {
                  Tiles tile_ = deserializedtile.tile[i];
                          //Do whatever you want with the data.
                          
                        Debug.Log(i + tile_.tile.ID);
                         
              }
           }
 }

I'm getting null reference exception on this line : Debug.Log(i + tile_.tile.ID); ObjectReference Not set to an instance of object

and here is my class holding the xml format information :

 using UnityEngine;
 using System.Collections;
 using System.Xml;
 using System.Xml.Serialization;
 
 [System.Serializable]
 public class Tiles
 {
     public class Tile
     {
 
         [XmlAttribute("posx")]
         public int posx = 0;
         [XmlAttribute("posy")]
         public int posy = 0;
         [XmlAttribute("posz")]
         public int posz = 0;
         [XmlAttribute("house")]
         public bool house = false;
         [XmlAttribute("isPZ")]
         public bool isPZ = false;
         [XmlAttribute("ID")]
         public int ID = 0;
 
 
     
 
         public class items
         {
             public class item
             {
 
                 [XmlAttribute("ID")]
                 public int item_ID = 0;
 
             }
 
 
         }
     }
     [XmlElement("tile")]
     public Tile tile;
 }

Thx for helping me out!