[BUG] UnityMainThreadDispatcher destroyed

I just hit this exception.

```
MissingReferenceException: The object of type 'UnityMainThreadDispatcher' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
UnityEngine.MonoBehaviour.StartCoroutine (System.Collections.IEnumerator routine) (at :0)
ImmersiveVRTools.Runtime.Common.UnityMainThreadDispatcher+<>c__DisplayClass2_0.b__0 () (at :0)
ImmersiveVRTools.Runtime.Common.UnityMainThreadDispatcher.Update () (at :0)
```

I had left Unity open in the background for a few hours, and this exception happened after the first change when I returned. Editor reloading is enabled, and I was not in play mode. Unfortunately I don't have any other information about the circumstances that triggered it.

Start at UnityMainThreadDispatcher, especially Update and the Enqueue callback shown in the stack trace, and inspect when the dispatcher is destroyed or reused. Reproduce the editor-reload scenario outside play mode if possible, then verify that the MissingReferenceException no longer occurs after leaving Unity open and making a change.

索引モデルが issue の本文から書いたものです。