Reporting Services :: SSRS Custom Data Processing Extension Error

I am using Custom Data Processing Extension to call a stored procedure. Iam getting following error when creating a dataset in report designer using the extension. I wrote the code in c#.

could not update a list of fields for the query. verify that you can connect to the data source and that your query syntax is correct.(Details-Object reference not set to an instance of an object.)

Here is my code

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Data;

[Code] .....