Arduino serial Read

Hi there ! I'm having a problem with some js code to read a stream from arduino. Here is the error:

 Error: System.NullReferenceException: Object reference not set to an instance of an object
   at (wrapper managed-to-native) System.IO.Ports.WinSerialStream:ClearCommError (int,uint&,System.IO.Ports.CommStat&)
   at System.IO.Ports.WinSerialStream.get_BytesToRead () [0x00000] in <filename unknown>:0 
   at System.IO.Ports.SerialPort.get_BytesToRead () [0x00000] in <filename unknown>:0 
   at System.IO.Ports.SerialPort.ReadExisting () [0x00000] in <filename unknown>:0 
   at (wrapper remoting-invoke-with-check) System.IO.Ports.SerialPort:ReadExisting ()
   at LectorSerial.LeerDatos () [0x00006] in Z:\home\apolak\Proyectos\Unity3D\Apuntador\Assets\Scenes\Prueba Encoder\LectorSerial.js:62 
 UnityEngine.Debug:Log(Object)
 LectorSerial:LeerDatos() (at Assets/Scenes/Prueba Encoder/LectorSerial.js:66)
 UnityEngine.EventSystems.EventSystem:Update()

I'm using this script:

 #pragma strict
 import System.IO.Ports;
 
  public static var stream : SerialPort;
  public var nombrePuerto = "/dev/ttyACM1";
 
 
  public var abroPuerto = false;
  public var cierro = false;
  
  function Start () 
  {
      CrearSerialPort();
  }
 
  function Update () 
  {
      AbrirConexion();
  }
 
  function CrearSerialPort()
  { 
      stream = new SerialPort ();
      stream.BaudRate = 115200;
      stream.PortName = nombrePuerto;
      stream.Parity = Parity.None;
      stream.DataBits = 8;
      stream.StopBits = StopBits.One;
  }
 
  function AbrirConexion()
  {
      if (abroPuerto && !stream.IsOpen) 
     {
         Debug.Log("Antes del Try");
         try
         {
             stream.Open (); //Abro el stream del puerto serial.
             stream.ReadTimeout = 1000;
             Debug.Log("Esta abierto: "+stream.IsOpen);
 
         }
         catch (ex)
         {
             Debug.Log("Error: "+ex.ToString());
         }
     }
 
     if (cierro) 
     {
         Debug.Log("Entro a Cierro");
         stream.Close ();
         abroPuerto = false;
     }
  }
 
  function LeerDatos()
  {
      try
      {
           var receivedData = "";
           receivedData = stream.ReadExisting().ToString();
       }
     catch (ex)
     {
         Debug.Log("Error: "+ex.ToString());
     }
 
  }
 
  function OnApplicationQuit()
  {
      stream.Close();
  }