How To Insert and Update into database table using LinqTemplate?

Question

I have a problem while creating a web application using LinqTemplate here is the explanation step wise....

(1) I have created a database Database1 with a table name UserTest with one column name UserId with (uniqidentifier,not null) datatype , username (nvarchar2(20),null)

(2) I have created a classLiberary Project where i have used LinqTemplate of subsonic for code Generation of DAL and i have created a partial class for insert and update query

namespace SubsonicExample
{
class UserTest
{
       public int Insert(UserTest user)
       {
           SubsonicExampleSystemDB db = new SubsonicExampleSystemDB();
         return  db.Insert.Into<UserTest>
               ( 
               x => x.UserId,
               x => x.Username
               ).Values(
               user.UserId,
               user.Username,
               ).Execute();
         }
         public int Update(UserTest user)
         {

           SubsonicExampleSystemDB db = new SubsonicExampleSystemDB();
           return db.Update<UserTest>().Set
                  (
                  x => x.Username == user.Username
                  ).Where(
                  x => x.UserId == user.UserId
                  ).Execute();
            }
}
}

(3) I have created a web project in same solution and created a page UserTest.aspx when I try to insert or create a new User or edit the existing User it throws an exception. In My Page There is a textBox and a button

protected void Button1_Click(object sender, EventArgs e)
 { 
   SubsonicExampleSystemDB db = new SubsonicExampleSystemDB();
    UserTest user = null;
    if (string.IsNullOrEmpty(Request.QueryString["Id"]))
     {   
      user.UserId = Guid.NewGuid();
      user.Username=Textbox1.Text;
     user.Insert(user);
     }
    else
    {
         Guid userId = new Guid(Request.QueryString["Id"].ToString());
         user= db.UserTests.FirstOrDefault(u => u.UserId == userId);
         user.Username= Textbox1.Text;
        user.Update(user);
    }
}

Now when I Build the project it build successfully and when i run the project it shows the UserTest.aspx page with a button & TextBox but when i Enter value and click on button it must insert the new guid value into the table but it throws an exception:

System.InvalidCastException: Object must implement IConvertible.
[InvalidCastException: Object must implement IConvertible.]
   System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider) +2560525
   System.Data.SqlClient.SqlParameter.CoerceValue(Object value, MetaType destinationType) +414
[InvalidCastException: Failed to convert parameter value from a Guid to a String.]

and when i try to update data in text box and click on Button it throws exception

System.NullReferenceException: Object reference not set to an instance of an object.
[NullReferenceException: Object reference not set to an instance of an object.]

This is my dal Classes

public partial class UserTest: INotifyPropertyChanging, INotifyPropertyChanged
{
        partial void OnLoaded();
        partial void OnValidate(System.Data.Linq.ChangeAction action);
        partial void OnCreated();

     public Test(){
         OnCreated();
     }

     #region Properties

        partial void OnUserIdChanging(Guid value);
        partial void OnUserIdChanged();

  private Guid _UserId;
  public Guid UserId { 
      get{
          return _UserId;
      } 
      set{
          this.OnUserIdChanging(value);
                this.SendPropertyChanging();
                this._UserId = value;
                this.SendPropertyChanged("UserId");
                this.OnUserIdChanged();
      }
  }

        partial void OnUserNameChanging(string value);
        partial void OnUserNameChanged();

  private string _UserName;
  public string UserName { 
      get{
          return _UserName;
      } 
      set{
          this.OnUserNameChanging(value);
                this.SendPropertyChanging();
                this._UserName = value;
                this.SendPropertyChanged("UserName");
                this.OnUserNameChanged();
      }
  }

        partial void OnOccupationChanging(Guid? value);
        partial void OnOccupationChanged();

  private Guid? _Occupation;
  public Guid? Occupation { 
      get{
          return _Occupation;
      } 
      set{
          this.OnOccupationChanging(value);
                this.SendPropertyChanging();
                this._Occupation = value;
                this.SendPropertyChanged("Occupation");
                this.OnOccupationChanged();
      }
  }
   partial void Onuser_usernameChanging(string value);
        partial void Onuser_usernameChanged();

  private string _user_username;
  public string user_username { 
      get{
          return _user_username;
      } 
      set{
          this.Onuser_usernameChanging(value);
                this.SendPropertyChanging();
                this._user_username = value;
                this.SendPropertyChanged("user_username");
                this.Onuser_usernameChanged();
      }
  }


        #endregion

        #region Foreign Keys
        #endregion


        private static PropertyChangingEventArgs emptyChangingEventArgs = new PropertyChangingEventArgs(String.Empty);
        public event PropertyChangingEventHandler PropertyChanging;
        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void SendPropertyChanging()
        {
            var handler = PropertyChanging;
            if (handler != null)
               handler(this, emptyChangingEventArgs);
        }

        protected virtual void SendPropertyChanged(String propertyName)
        {
            var handler = PropertyChanged;
            if (handler != null)
                handler(this, new PropertyChangedEventArgs(propertyName));
        }

 }