Hello,
I am not sure if I understand your question, but I will try...
According to our documentation, you should test eventArgs.Exception before accessing eventArgs.Vtq. If eventArgs.Exception is non-null, then it contains information about the problem, and eventArgs.Vtq is null, so you should not dereference it.
When eventArgs.Exception is null, then it is guaranteed that eventArgs.Vtq. is not null. But, eventArgs.Vtq.Value may still be null, if the quality (eventArgs.Vtq.Quality) is bad, or even if it is good but the OPC server has sent a null (VT_EMPTY) value.
Best regards

Hello,
I cannot give definitive answer, because your post does not state the details about the exception (call stack etc.), or which object is null but you think it should not be.
But I have looked at your code and made some general observations, though:

The Main method just subscribes, and the method then immediately exits. This does not look right - I am sure you want to stay subscribed for some time. It could be shutting down the program while still subscribed, which can lead to problems. You need to have some logic regarding how long you want the program to stay subscribed, and only exit the Main method when you are done (and, proper clean up, by unsubscribing, is highly recommended).
OPC Servers can send notifications with Bad quality at any time, due to conditions in the underlying system, or (with some servers) even initially, before the first data is available. When this happens, the Value portion of the DAVtq object (the Vtq property of the notification event arguments) does not contain any valid data, and is therefore null. But your code is doing eventArgs.Vtq.Value.ToString(), without checking it first. This is not right.

I hope this helps.