Only Id becomes Null when loading nodes in C#XML

<AppSettings>
  <FileFolder>C:\</FileFolder>
  <Sky>0</Sky>
  <Users>
    <UserId="User0001">
      <Key></Key>
      <Address>D:\C#Create\Face Mark_Free Material\img038.png</Address>
    </User>
    <UserId="User0002">
      <Key>2</Key>
      <Address/>
    </User>
    <UserId="User0003">
      <Key>3</Key>
      <Address/>
    </User>
  </Users>
</AppSettings>

 [Serializable()]
public class AppSettings
{
    public string FileFolder {get;set;}
    public byte SKy { get; set; } // Array number of labels
    // User Information
    public List <User>Users {get;set;}
    public class User
    {
        public string Id { get; set; } // ID
        public string key { get; set; } // username
        public string address {get; set; } // user image address
    }

    // Instances of the Settings class
    NonSerialized()
    private static AppSettings_instance;
    [System.Xml.Serialization.XmlIgnore]
    public static AppSettings Instance
    {
        get
        {
            // If_instance is null, create a new instance
            if(_instance==null)
                _instance=new AppSettings();

            return_instance;
        }
        set {_instance=value;}
    }
}

private void MainForm_Load(object sender, EventArgse)
{
    // -- Abbreviated --
    foreach (AppSettings.UserLabel in AppSettings.Instance.Users) // Load Saved User Labels
    {
        if(userLabel.Key!="")
        { 
            UserList.Items.Add(userLabel.Key);
            if(UCount==KeyNum)UserName.Text=userLabel.Key;//Key of the saved UserId
        }
        else
        {
            UserList.Items.Add(userLabel.Id);
            if(UCount==KeyNum)
            {
                UserName.Text=userLabel.Id;
                UserPicture.BackgroundImage=Image.FromFile(userLabel.Address); // Saved UserId Address
            }
        }
    }
}