Cannot open Sales invoice journal

Hi there, I'm a development partner. One question about the latest SDK - 2015.1. I'm developing some interface and  need to open sales journal. I found a good example in SDK. So, I tried to follow it and it worked for a while. But out of blue, I got an error message:

An unhandled exception of type 'System.NullReferenceException' occurred in Sage_SA.Domain.Utility.dll
Additional information: Object reference not set to an instance of an object.

My original code is:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Windows;
using System.Windows.Forms;
using Simply.Domain.Utility;
using SimplySDK;
using SimplySDK.Support;
using SimplySDK.GeneralModule;
using SimplySDK.PayableModule;
using SimplySDK.ReceivableModule;
using SimplySDK.PayrollModule;
using SimplySDK.InventoryModule;
using SimplySDK.ProjectModule;
using System.Data;
using System.Threading;


public class clSage

           SalesJournal salJourn;


 public void OpenSalJournal()
            salJourn = SDKInstanceManager.Instance.OpenSalesJournal();

So, when I enter this method OpenSalJournal() - I get this error message.

Please, let me know if you need more info. Thank you in advance!