Why this code don't work,when i want run this code vwd 2008 express show me this error:

Object reference not set to an instance of an object.

Line 73: kom.Parameters.Add("@subcategories", SqlDbType.Text).Value = s_categoreis.SelectedItem.ToString();

This is my ascx file:

 <asp:ListBox ID="categories" runat="server" Height="380px" CssClass="kat" AutoPostBack="true" DataSourceID="SqlDataSource1" 
        DataTextField="Categories" DataValueField="ID" 
        onselectedindexchanged="kategorije_SelectedIndexChanged"></asp:ListBox>


     <asp:Button ID="Button1" CssClass="my" runat="server" Text="click" 
            onclick="Button1_Click" />

        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate> <asp:ListBox ID="s_categoreis" CssClass="pod" Height="150px" Enabled="true" runat="server"></asp:ListBox></ContentTemplate>
           <Triggers>
           <asp:AsyncPostBackTrigger ControlID="categories" EventName="SelectedIndexChanged" />

           </Triggers>
        </asp:UpdatePanel>


    <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
        ConnectionString="<%$ ConnectionStrings:estudent_piooConnectionString %>" 
        SelectCommand="SELECT [ID], [Categories] FROM [categories]">
    </asp:SqlDataSource>


and this is my ascx.cs:


 SqlConnection veza;
    SqlCommand kom = new SqlCommand();
    SqlParameter par1 = new SqlParameter();
    SqlParameter par2 = new SqlParameter();
    SqlParameter par3 = new SqlParameter();
    SqlParameter par4 = new SqlParameter();
    SqlParameter par5 = new SqlParameter();
    SqlParameter par6 = new SqlParameter();
    SqlParameter par7 = new SqlParameter();
    SqlParameter par8 = new SqlParameter();
    SqlParameter par9 = new SqlParameter();
    protected void Page_Load(object sender, EventArgs e)
    {



            Listapod_kategorije(1);

    }
    protected void kategorije_SelectedIndexChanged(object sender, EventArgs e)
    {
        Listapod_kategorije(Convert.ToInt32(kategorije.SelectedValue));

    }
    private void Listapod_kategorije(int broj) {

        SqlDataSource ds = new SqlDataSource();
        ds.ConnectionString = ConfigurationManager.ConnectionStrings["estudent_piooConnectionString"].ConnectionString;
        ds.SelectCommand = "Select * from pod_kategorije where kat_id=" + broj;
        pod_kategorije.DataSource = ds;
        pod_kategorije.DataTextField = "pkategorija";
        pod_kategorije.DataValueField = "ID";
        pod_kategorije.DataBind();


    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Guid jk = new Guid();

        object datum = DateTime.Now;
        veza = new SqlConnection(@"server=85.94.76.170\PADME; database=estudent_pioo;uid=pioo;pwd=1234567");

            Random broj = new Random();
            int b_kor = broj.Next(1, 1000);
            kom.Parameters.Add("@text", SqlDbType.Text).Value = str;
            kom.Parameters.Add("@user", SqlDbType.UniqueIdentifier).Value = jk;

            kom.Parameters.Add("@date", SqlDbType.DateTime).Value = datum;
            kom.Parameters.Add("@visits", SqlDbType.Int).Value = 0;
            kom.Parameters.Add("@answers", SqlDbType.Int).Value = 0;
            kom.Parameters.Add("@username", SqlDbType.Text).Value = "unknown_" + b_kor.ToString(); ;
            kom.Parameters.Add("@categories", SqlDbType.Text).Value = categories.SelectedItem.ToString();
            kom.Parameters.Add("@sub_categories", SqlDbType.Text).Value = s_categoreis.SelectedItem.ToString();
            veza.Open();
            kom.ExecuteNonQuery();
            veza.Close();
            Response.Redirect("default.aspx");