while running this code i get an error said "Object reference not set to an instance of an object"

protected void btnSave_Click(object sender, EventArgs e)
        {
            SqlCommand cmd = new SqlCommand("exec insertMission @fonction,@service,@missionObj,@excpecteRslt,@startDate,@returnDate,@District_code,@codeFinance,@empno,
@transNum", con);
            cmd.Parameters.AddWithValue("@fonction",txtFonction.Text);
            cmd.Parameters.AddWithValue("@service",txtService.Text);
            cmd.Parameters.AddWithValue("@missionObj",txtObject.Text);
            cmd.Parameters.AddWithValue("@excpecteRslt",txtResult.Text);
            cmd.Parameters.AddWithValue("@startDate",txtStartDate.Text);
            cmd.Parameters.AddWithValue("@returnDate",txtEndDate.Text);
            cmd.Parameters.AddWithValue("@District_code",DropDownListDistr.SelectedValue);
            cmd.Parameters.AddWithValue("@codeFinance",DropDownListFinance.SelectedValue);
            cmd.Parameters.AddWithValue("@empno", DropDownList1.SelectedValue);
            cmd.Parameters.AddWithValue("@transNum",DropDownListTrans.SelectedValue);           
            con.Close();
            con.Open();
            cmd.ExecuteNonQuery();
        }