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

public string InsertDB(ShortCodeBO objShortCodeBO)
       {

           string message = string.Empty;
           try
           {

               string CommonDatabaseName = ConfigurationSettings.AppSettings["CommonDatabaseName"].ToString();
               string ConName = CommonDatabaseName;

               //ConnectionString_Initialize(StoredProcedure.InsertDB, ConName);
               Initialize(StoredProcedure.InsertDB);
               UserCmd.Parameters.Clear();
               UserDB.AddInParameter(UserCmd, Parameter.UserName, DbType.String,objShortCodeBO.UserName);
               UserDB.AddInParameter(UserCmd, Parameter.Password, DbType.String,objShortCodeBO.Password);
               UserDB.AddInParameter(UserCmd, Parameter.FirstName, DbType.String,objShortCodeBO.FirstName);
               UserDB.AddInParameter(UserCmd, Parameter.LastName, DbType.String,objShortCodeBO.LastName);
               UserDB.AddOutParameter(UserCmd, Parameter.ERROR, DbType.String, 100);

               UserDB.ExecuteNonQuery(UserCmd);
               message = (string)UserCmd.Parameters["@ERROR"].Value;
           }
           catch (SqlException)
           {
               throw;
           }
           catch (Exception)
           {
               throw;
           }
           return message;
       }