have error on my database script

 i have some trouble on my program
 
 it error "NullReferenceException: Object reference not set to an instance of an object
 MapLock_Chapter1.Start () (at Assets/Script/MapLock_Chapter1.js:28)"
 
 this is my code that error 

   

        #pragma strict
             import System.Data;
             import Mono.Data.SqliteClient;
             
             
                 private var connection : String;
                 private var dbcon : IDbConnection;
                 private var dbcmd : IDbCommand;
                 private var reader : IDataReader;
                 var nextLevel:String;
                 var texturelock : Texture2D;
                 var textureunlock : Texture2D;
                 var QuitButton:boolean;
                 var chapterUnlocked : boolean;
                 var chapterlocked : boolean;
                 var needitem:GUIText;
                 var text:String = " Gather 100 Diamond to unlock ";
                 
             
                 function Start(){
                 connection = "URI=file:" + "23.s3db"; // we set the connection to our database
                 dbcon = new SqliteConnection(connection);
                 dbcmd = dbcon.CreateCommand(); // create empty command
                 var cmdSql = "SELECT diamond FROM test";
                 dbcmd.CommandText = cmdSql; // fill the command
                 dbcon.Open();
                 reader = dbcmd.ExecuteReader(); // execute command which returns a reader
                 while( reader.Read() ) {
                 var pdiamond : int = int.Parse(reader["diamond"].ToString()); 
                 chapterUnlocked = ( pdiamond > 99);
                      chapterlocked = ( pdiamond < 100);
                     print(pdiamond);
                     }
                 }
             
             function Update () {
             
             }
             
             function OnGUI ()
             {
             if (chapterlocked)
             { guiTexture.texture = texturelock;
               needitem.text = text; }
             
             if (chapterUnlocked)
             { guiTexture.texture = textureunlock; }
             
             }
             
             function OnMouseUp ()
             {
             if(QuitButton)
             { Application.Quit(); }
             
             else
             { Application.LoadLevel(nextLevel); }
             }
 
 
 plz help me
 
 thank you.