SqlDataAdapter.Fill() problem

Dear All,
I'm filling a sql data adapter with a data table , this is y code
DataTable ds = new DataTable();
//SqlConnection sqlconDS = new SqlConnection( System.Configuration.ConfigurationManager.AppSettings["shcon"] );
SqlCommand sqlthrdcom = new SqlCommand();
sqlthrdcom.Connection = sqlcon2;
sqlthrdcom.CommandType = CommandType.StoredProcedure;
sqlthrdcom.CommandText = "SP_IncomingSMS_InSender";
sqlthrdcom.Parameters.Clear();
sqlthrdcom.Parameters.AddWithValue("@serviceID", s_id);

SqlDataAdapter sqlADA = new SqlDataAdapter(sqlthrdcom);
sqlADA.Fill(ds);

Yet, i have a problem which is the code is running successfuly...but in random times it throws the exception "Object Refrence not set to an instance of value"
This exception has been thrown when the adapter is filled with data..that's what i got from tracing the code.
this previous code is the begining of a large thread...however my task is to know why this exception is thrown
anyone can help i'll be grateful.
Thanks in advance.