Fix for a nullref thrown when PSES calls ForcePSEventHandling

[TylerLeonhardt]

In PowerShell Editor Services, we have a private contract with PSRL with a method called ForcePSEventHandling found in:

• PSRL
• PSES

Due to the multi-threaded nature of PSES, we are calling ForcePSEventHanding before the _forceEventWaitHandle was initialized (which happens in DelayedOneTimeInitialize() thus causing a null reference exception.

This PR moves the initialization to the field declaration which fixes the issue we were seeing (PSES crashing).

This should be low risk as this is only used by PSES and they are a private contract.

[TylerLeonhardt]

There are some greater questions that can be asked on the overall design of this contract, but this at least mitigates the issue for now.

cc @SeeminglyScience

[daxian-dbw]

Would changing ForcePSEventHandling to _singleton?._forceEventWaitHandle?.Set(); solve the PSES problem too? (syntax needs to change to compile with 4.6.1 though)

[TylerLeonhardt]

Thought about that but we want that _forceEventWaitHandle to be set when we call the method. It probably doesn't make a huge difference, but this PR keeps the idea consistent of "when this method is called, it sets this AutoResetEvent" rather than it being best effort.

[daxian-dbw]

I'm fine with this change.

[TylerLeonhardt]

I consulted heavily with @SeeminglyScience on this - who originally did the ground work. I'd like to give him a chance to sign off so hold off on merging until he gives a thumbs up.

[SeeminglyScience]

LGTM!

[TylerLeonhardt]

I don't see a problem with that, @daxian-dbw 👍 LGTM