Crash (null pointer) when calling functions without a valid solution

[PMeira]

Many functions require a valid/initialized state or otherwise will crash. For example, any function that calls cktelement.GetCurrents should crash.

This happens both in official COM and C-API, so it's not urgent.

We need to select the proper places to check this. Checking ActiveCircuit.IsSolved might be enough for the general circuit, checking if cktelement.NodeRef is not null seems enough for individual circuit elements.

[PMeira]

I noticed the Topology interface can also crash without a valid solution, while testing it to reply to this thread: https://sourceforge.net/p/electricdss/discussion/beginners/thread/9add05221d/

[PMeira]

ActiveCircuit.IsSolved needs a converged solution, which is not ideal.
I added some checks for NodeV (for the system), probably will need more in both the interface and internal code. NodeRef (each CktElement has an array) might be useful.

[PMeira]

I haven't received any crash reports recently. v0.12 adds more error messages, let's see how it goes.

[PMeira]

I noticed some reports (DSS commands show, export) can also segfault, so we should check those too before closing this even if the basic API calls are OK-ish now.