InvalidOperationException in MixedRealityToolkitFiles when editor was reloading scripts

[keveleigh]

Describe the bug

Specifically, after I switched branches and Unity was reloading, I saw the following print out in the editor:

InvalidOperationException: Collection was modified; enumeration operation may not execute.
System.Collections.Generic.Dictionary`2+Enumerator[TKey,TValue].MoveNext () (at <df7127ba07dc446d9f5831a0ec7b1d63>:0)
Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitFiles.TryUnregisterModuleFolder (System.String folder) (at Assets/MixedRealityToolkit/Utilities/Editor/Setup/MixedRealityToolkitFiles.cs:220)
Microsoft.MixedReality.Toolkit.Utilities.Editor.MixedRealityToolkitFiles+AssetPostprocessor.OnPostprocessAllAssets (System.String[] importedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromAssetPaths) (at Assets/MixedRealityToolkit/Utilities/Editor/Setup/MixedRealityToolkitFiles.cs:91)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <df7127ba07dc446d9f5831a0ec7b1d63>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <df7127ba07dc446d9f5831a0ec7b1d63>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <df7127ba07dc446d9f5831a0ec7b1d63>:0)
UnityEditor.AssetPostprocessingInternal.PostprocessAllAssets (System.String[] importedAssets, System.String[] addedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromPathAssets) (at C:/buildslave/unity/build/Editor/Mono/AssetPostprocessor.cs:141)

To reproduce

I've seen this a few times, so here are what I think the most likely repro steps are:

1. Switch branches while the project is open in Unity (or maybe even just edit a script so Unity reloads something) (it might be specific to a folder being created / removed)
2. Re-focus the editor
3. Allow it to reload assets
4. See exception in console

Screenshots

Your Setup (please complete the following information)

• Unity Version [e.g. 2018.3.11f1]: 2018.4.6f1
• MRTK Version [e.g. v2.0]: mrtk_development

[Alexees]

Kind of tried fixing this with #5624, but @wiwei wanted to wait for a different PR, but I'm not entirely sure if it's in and did not fix this, or is still not...

[stale]

This issue has been marked as stale by an automated process because it has not had any recent activity. It will be automatically closed in 30 days if no further activity occurs. If this is still an issue please add a new comment with more recent details and repro steps.

[stale]

This issue has been closed by an automated process because it is stale. If this is still an issue please add a new comment with more recent details and repro steps.