dropdownList doesn't displays selected value - asp.net

Question

I am new at asp.net. I have a dropdownList which displays all categories for a specific article. The problem is that when I want to edit an article it doesn't displays the value that exists as default selected in dropdownList.
protected void datalist2_OnItemCreated(object sender, DataListItemEventArgs e)
{
if (e.Item.ItemType == ListItemType.EditItem)
{
DropDownList drpdKategoria = e.Item.FindControl("drpdKategoria") as DropDownList;
SqlConnection con = new SqlConnection(connection);
string Qry = "select * from kategoria";
SqlDataAdapter da = new SqlDataAdapter(Qry, con);
DataSet ds = new DataSet();
con.Open();
da.Fill(ds);
drpdKategoria.DataSource = ds;
drpdKategoria.DataValueField = "id";
drpdKategoria.DataTextField = "emertimi";
drpdKategoria.DataBind();
drpdKategoria.SelectedValue = drpdKategoria.Items.FindByText(Qry).Value;
//drpdKategoria.Items.FindByValue(string val).Selected = true;
con.Close();
con.Dispose();
ds.Dispose();
da.Dispose();
}
}
EditArtikull.aspx
<asp:Label ID="Label5" runat="server" style="font-weight: 700">Kategoria</asp:Label>
<fieldset>
<asp:DropDownList ID="drpdKategoria" runat="server" AutoPostBack="false"></asp:DropDownList>
</fieldset>
<br/>
ERROR:SystemNullReference Exception {"Object reference not set to an instance of an object."}

Answer 1

This line probably isn't going to find anything:
drpdKategoria.Items.FindByText(Qry)
Since Qry is a SQL statement:
string Qry = "select * from kategoria";
And I'm assuming that the display values in your DropDownList aren't SQL queries. Thus, when you call .Value on that first line, you're trying to de-reference something that isn't found (which is null), hence the error.
What item are you actually trying to find? If you want to select a default item, you need to be able to identify that item in some way. In your data that would be either by a known id value or a known emertimi value. For example, if you have a known emertimi value, it would be:
drpdKategoria.SelectedValue = drpdKategoria.Items.FindByText("some known value").Value
To make this a little more robust, you probably want to add some null checking. Something like this:
var defaultValue = drpdKategoria.Items.FindByText("some known value");
if (defaultValue != null)
drpdKategoria.SelectedValue = defaultValue.Value