NRE in `BuildCheckManager.ProcessProjectEvaluationStarted`

[rainersigwald]

Hit this failure in an unrelated PR: https://dev.azure.com/dnceng-public/public/_build/results?buildId=853422&view=ms.vss-test-web.build-test-results-tab&runId=22164632&resultId=100110&paneView=attachments

(0,0):  error MSB4166: Child node "1" exited prematurely. Shutting down. Diagnostic information may be found in files in "C:\Users\VssAdministrator\AppData\Local\Temp\3tfdpukg.t1q\MSBuildTemp\" and will be named MSBuild_*.failure.txt. This location can be changed by setting the MSBUILDDEBUGPATH environment variable to a different directory.
Microsoft.Build.Exceptions.InternalLoggerException: The build stopped unexpectedly because of an unexpected logger failure.
 ---> Microsoft.Build.Exceptions.InternalLoggerException: The build stopped unexpectedly because of an unexpected logger failure.
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at System.Collections.Generic.Dictionary`2.Add(TKey key, TValue value)
   at Microsoft.Build.Experimental.BuildCheck.Infrastructure.BuildCheckManagerProvider.BuildCheckManager.ProcessProjectEvaluationStarted(ICheckContext checkContext, String projectFullPath) in /_/src/Build/BuildCheck/Infrastructure/BuildCheckManagerProvider.cs:line 544
   at Microsoft.Build.Experimental.BuildCheck.Infrastructure.BuildCheckBuildEventHandler.HandleProjectEvaluationStartedEvent(ProjectEvaluationStartedEventArgs eventArgs) in /_/src/Build/BuildCheck/Infrastructure/BuildCheckBuildEventHandler.cs:line 93
   at Microsoft.Build.Experimental.BuildCheck.Infrastructure.BuildCheckBuildEventHandler.<.ctor>b__5_2(BuildEventArgs e) in /_/src/Build/BuildCheck/Infrastructure/BuildCheckBuildEventHandler.cs:line 33
   at Microsoft.Build.Experimental.BuildCheck.Infrastructure.BuildCheckBuildEventHandler.HandleBuildEvent(BuildEventArgs e) in /_/src/Build/BuildCheck/Infrastructure/BuildCheckBuildEventHandler.cs:line 59
   at Microsoft.Build.Experimental.BuildCheck.Infrastructure.BuildCheckConnectorLogger.EventSource_AnyEventRaised(Object sender, BuildEventArgs e) in /_/src/Build/BuildCheck/Infrastructure/BuildCheckConnectorLogger.cs:line 62
   at Microsoft.Build.BackEnd.Logging.EventSourceSink.RaiseAnyEvent(BuildEventArgs buildEvent) in /_/src/Build/BackEnd/Components/Logging/EventSourceSink.cs:line 388

https://github.com/dotnet/msbuild/blob/cdcdacfd8a89bddd88fcdd3e63f70c7faf57a48b/src/Build/BuildCheck/Infrastructure/BuildCheckManagerProvider.cs#L544

Maybe a race accessing that dictionary?