ImmersiveVRTools.Common.Runtime: UnityMainThreadDispatcher can't be initialized on worker thread

Just got hit by an exception where code tried to get the `.gameObject` property from a worker thread. It was during `SafeInvoke(HotReloadFailed, changesAwaitingHotReload);` triggering `ProjectWindowReloadStatusIndicatorDrawer.OnHotReloadFailed` which in turn accesses `UnityMainThreadDispatcher.Instance` which in `SingletonBase` will try to access stuff that should only be accessed on the main thread in case there is no singleton instance. Sorry for oral account of the callstack but my debugger got detached when I was poking around and CBA to repro. 😅

Anyway, maybe the main thread dispatcher needs to eaglery be inited from the main thread (RuntimeInitializeOnLoad?) if its means to be accessed from a worker, or just use another singleton implementation that doesn't hit Unity's API.

Start with the reported call chain from SafeInvoke(HotReloadFailed, changesAwaitingHotReload) through ProjectWindowReloadStatusIndicatorDrawer.OnHotReloadFailed, UnityMainThreadDispatcher.Instance, and SingletonBase. Determine which initialization path accesses Unity APIs from the worker thread. Done means the reported hot-reload failure no longer triggers a worker-thread Unity API exception, with the relevant behavior verified in Unity.

Vom Indexierungsmodell aus dem Issue-Text verfasst.