Data set null problem

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C#3.5

ASP.NET4

dear sir i want to insert the data in the tables in the database through the asp.net
but data is not inserted in the database...
i have written the following code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;

public partial class Registration : System.Web.UI.Page
{
    SqlConnection con = new SqlConnection("Data Source=LAPTOP-0800NRBO\\SQL;Initial Catalog=HealthCaareAdvisor;User ID=sa;Password=nitinmca");
    SqlCommand cmd;
    DataSet ds,ds1,ds2,ds3 = new DataSet();
    SqlDataAdapter sda,sda1,sda2,sda3;
    public void show()
    {
        //ds = null;
        sda = new SqlDataAdapter("select Gender from Registration", con);
        sda.Fill(ds);
        DrdpGender.DataSource = ds;
        DrdpGender.DataTextField = ds.Tables[0].Columns[0].ToString();
        DrdpGender.DataValueField = ds.Tables[0].Columns[0].ToString();
       DrdpGender.DataBind();
       // ds1 = null;
        sda1 = new SqlDataAdapter("select city from Registration", con);
        sda1.Fill(ds1);
        DrdpCity.DataSource = ds1;
        DrdpCity.DataTextField = ds.Tables[0].Columns[1].ToString();
        DrdpCity.DataValueField = ds.Tables[0].Columns[1].ToString();
        DrdpCity.DataBind();
       // ds2 = null;
        sda2 = new SqlDataAdapter("select qualification from Registration", con);
        sda2.Fill(ds2);
        DrdpQualification.DataSource = ds2;
        DrdpQualification.DataTextField = ds.Tables[0].Columns[2].ToString();
        DrdpQualification.DataValueField = ds.Tables[0].Columns[2].ToString();
        DrdpQualification.DataBind();
       // ds3 = null;
        sda3 = new SqlDataAdapter("Select Occupation  from Registration", con);
        sda3.Fill(ds3);
        DrdpOccupation.DataSource = ds3;
        DrdpOccupation.DataTextField = ds.Tables[0].Columns[3].ToString();
        DrdpOccupation.DataValueField = ds.Tables[0].Columns[3].ToString();
        DrdpOccupation.DataBind();        
    }

    public void fillgrid()
    {
        con.Open();
        DataSet dsGridview = new DataSet();
        SqlDataAdapter sdaGridview = new SqlDataAdapter("select * from Registration", con);
        sdaGridview.Fill(dsGridview);
        GridView1.DataSource = dsGridview;
        GridView1.DataBind();
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            show();
            fillgrid();
        }
    }

    protected void BtnRegister_Click(object sender, EventArgs e)
    {
        try
        {
            con.Open();
            string str = "insert into Registration values(@RegID,@Name,@LName,@DOB,@gender,@EmailP,@EmailS,@ContactP,@ContactS,@Address,@City,@Qualification,@Occupation,@Password)";
            cmd.Parameters.AddWithValue("@RegID", TxtregID.Text);
            cmd.Parameters.AddWithValue("@Name", Txtname.Text);
            cmd.Parameters.AddWithValue("@LName", TxtLname.Text);
            cmd.Parameters.AddWithValue("@DOB", DrdpGender.SelectedItem.Text);
            cmd.Parameters.AddWithValue("@EmailP", txtEmailP.Text);
            cmd.Parameters.AddWithValue("@EmailS", TxtEmailS.Text);
            cmd.Parameters.AddWithValue("@ContactP", TxtContactP.Text);
            cmd.Parameters.AddWithValue("@ContactS", TxtContactS.Text);
            cmd.Parameters.AddWithValue("@Address", TxtAddr.Text);
            cmd.Parameters.AddWithValue("@City", DrdpCity.SelectedItem.Text);
            cmd.Parameters.AddWithValue("@Qualification", DrdpQualification.SelectedItem.Text);
            cmd.Parameters.AddWithValue("@Occupation", DrdpOccupation.SelectedItem.Text);
            cmd.Parameters.AddWithValue("@Password", txtpass);

            cmd.ExecuteNonQuery();
            show();
            fillgrid();
            Response.Write("Record inserted");
        }

        catch (SqlException ex)
        {
            Response.Write("sorry Exception occured" + ex.Message);
        }
        finally
        {
            con.Close();
        }
    }
}

but it is showing the follwoing exception error...
Value cannot be null.
Parameter name: dataSet

here the code snippet that is generating exception:

 //ds = null;
Line 19:         sda = new SqlDataAdapter("select Gender from Registration", con);
Line 20:         sda.Fill(ds);
Line 21:         DrdpGender.DataSource = ds;
Line 22:         DrdpGender.DataTextField = ds.Tables[0].Columns[0].ToString();

What I have tried:

i have tried the dropdown list databinding but not result