Dataset dosn't work in asp.net webforms

[Ho$$@M_Sh2awa]

I made an online application to store employees information in MS SQL,I used Disconnected model and I made my class for "DataSet configuration" and call method in it in The "Form_Load" event. This is the class

public static string constr = ConfigurationManager.ConnectionStrings["cnn1"].ConnectionString;

public static DataSet GetDataSet(string stored_name, string table_name, params SqlParameter[] prmarr)
{
    SqlConnection con = new SqlConnection(constr);
    SqlCommand cmd = new SqlCommand(stored_name, con);
    foreach (SqlParameter prm in prmarr)
    {
        cmd.Parameters.Add(prm);
    }
    cmd.CommandType = CommandType.StoredProcedure;
    SqlDataAdapter da = new SqlDataAdapter(cmd);
    DataSet ds = new DataSet();
    da.Fill(ds, table_name);

    return ds;

I set the Connection string in "Web.Config"

Till now all things is ok but when I insert data to "DataSet" Error message appear

Object reference not set to an instance of an object.
Line 42: DataRow drnew = ds.Tables["x"].NewRow();

this is the code I Type in insert Button

DataRow drnew = ds.Tables["x"].NewRow();
drnew["First_Name"] = txtfname.Text;
drnew["Last_Name"] = txtlname.Text;
drnew["Country"] = drpboxcoun.SelectedValue;
drnew["Email"] = txtemail.Text;
drnew["Experience_Years"] = txtexper.Text;
drnew["Current_Salary"] = txtcursal.Text;
drnew["Expected_Salary"] = txtexpsal.Text;

ds.Tables["x"].Rows.Add(drnew);

I check the types compatible with fields in my database. so where is the problem !!

[Jefferson_1]

did you initialize the dataset before using it?

[Sulaiman_1]

Have to initialize the dataset or directly DataTable with the column name and its type.

 DataTable dt = new DataTable();
dt.Columns.Add("UserId", typeof(Int32));
dt.Columns.Add("UserName", typeof (string));
dt.Columns.Add("Education", typeof (string));
dt.Columns.Add("Location",typeof(string));

DataRow dtrow = dt.NewRow();    // Create New Row
dtrow["UserId"] = 1;            //Bind Data to Columns
dtrow["UserName"] = "SureshDasari";
dtrow["Education"] = "B.Tech";
dtrow["Location"] = "Chennai";
dt.Rows.Add(dtrow);

gvDetails.DataSource = dt;
gvDetails.DataBind();