Error: - Object reference not set to an instance of an object.

Question

 public string AcceptDetails(string ISBN, string DateOrder, string CustName, string CustAddr1, string CustAddr2, string CustCity, string CustState, string OrdBy, string OrdStat, string CardType, string CardNum)
        {
            string OrderNo;
            string error;
            error = "";
            OrderNo = GenerateOrder();
            string InsStr;
            InsStr = "insert into dtorders values (@IN, @ON, @DO, @CN, @CA1, @CA2, @CC, @CS, @OB, @ST, @CT, @CNN)";
            try
            {
                SqlConnection conn = new SqlConnection("server=HOME-1E79531B32;database=DTDB;trusted_connection=yes");
                SqlCommand cmd1 = new SqlCommand(InsStr,conn);
                SqlDataAdapter dad1 = new SqlDataAdapter(cmd1);
                dad1.InsertCommand.Parameters.Add("@IN", SqlDbType.Char, 10).Value = ISBN;
                dad1.InsertCommand.Parameters.Add("@ON",SqlDbType.Char, 5).Value = OrderNo;
                dad1.InsertCommand.Parameters.Add("@DO", SqlDbType.DateTime, 8).Value = Convert.ToDateTime(DateOrder).Date;
                dad1.InsertCommand.Parameters.Add("@CN", SqlDbType.VarChar, 50).Value = CustName;
                dad1.InsertCommand.Parameters.Add("@CA1", SqlDbType.VarChar, 50).Value = CustAddr1;
                dad1.InsertCommand.Parameters.Add("@CA2", SqlDbType.VarChar, 50).Value = CustAddr2;
                dad1.InsertCommand.Parameters.Add("@CC", SqlDbType.VarChar, 20).Value = CustCity;
                dad1.InsertCommand.Parameters.Add("@CS", SqlDbType.VarChar, 10).Value = CustState;
                dad1.InsertCommand.Parameters.Add("@OB", SqlDbType.VarChar, 50).Value = OrdBy;
                dad1.InsertCommand.Parameters.Add("@ST", SqlDbType.VarChar, 20).Value = OrdStat;
                dad1.InsertCommand.Parameters.Add("@CT", SqlDbType.Char, 10).Value = CardType;
                dad1.InsertCommand.Parameters.Add("@CNN", SqlDbType.VarChar, 20).Value = CardNum;

                if (conn.State == ConnectionState.Closed)
                {
                    conn.Open();
                }
                dad1.InsertCommand.ExecuteNonQuery();
                conn.Close();

            }

            catch(Exception E1)
            {
                error = E1.Message;
            }

            string result;
            if (error.Length != 0)
            {
                result = "Record not Inserted due to the following reason: \n" + error;
            }

            else
            {
                result = "Record Inserted!!";
            }
            return result;
        }

Error: - Object reference not set to an instance of an object.