Guard against registry values that disappear mid-read

[dcalonego]

Three null dereferences that can each take down the plugin, plus one compiler warning.

The races

HWiNFO rewrites the VSB key on every polling cycle. A value name returned by GetValueNames() can already be gone by the time GetValue() runs a moment later.

GetSensorType cast the result to string and immediately called Trim():

var value = (string)key.GetValue(VALUE_REGISTRY_NAME + index);
var unit = value.Trim()...   // NullReferenceException

The cast yields null rather than throwing, so the exception surfaces on the next line and propagates out of Load().

GetId has the same race on the same value, via .ToString().

UpdateValues dereferenced _key with no check. Dispose() sets _key = null, so an update cycle overlapping a Close() throws inside FanControl's update loop.

On sensor ids

GetId now falls back to an empty unit. That is the same string the existing ?? string.Empty already produces for a value with no unit, so ids stay byte-identical and references stored in user configurations keep resolving. This mattered enough to double-check, since a changed id silently orphans a fan curve.

Also

Drops the unused exception variable in IsActive(). master currently builds with one warning:

HWInfoRegistry.cs(35,30): warning CS0168: The variable 'exception' is declared but never used

After this change the project builds clean.

Testing

Built against master, exercised through a harness against a live HWiNFO instance: 47 sensors, 15 update cycles, 0 exceptions. The races themselves are timing-dependent and I did not manage to reproduce them on demand — these are guards against a failure mode visible in the code, not a fix for an observed crash.

[Rem0o]

Update() after Close() is technically impossible, as it is done under lock upstream.

[dcalonego]

Removed, thanks. I inferred that race from reading the plugin in isolation, with no visibility into the locking upstream.

The other two guards are a different race: GetValueNames() and GetValue() are separate calls, so GetValue can return null for a name listed a moment earlier and GetSensorType calls Trim() on it. Happy to drop those too if you'd rather not carry the check.