CI failure in Microsoft.Extensions.Hosting.Unit.Tests on OSX

[tarekgh]

=== TEST EXECUTION SUMMARY ===
Microsoft.Extensions.Hosting.Unit.Tests Total: 81, Errors: 0, Failed: 0, Skipped: 0, Time: 3.260s

https://helix.dot.net/api/2019-06-17/jobs/da05d240-7fec-4d94-bff9-88da1f0983d4/workitems/Microsoft.Extensions.Hosting.Unit.Tests/console

Unhandled Exception:
System.NullReferenceException: Object reference not set to an instance of an object
at System.IO.FileSystemWatcher.RunningInstance.FileSystemEventCallback(IntPtr streamRef, IntPtr clientCallBackInfo, IntPtr numEvents, Byte** eventPaths, FSEventStreamEventFlags* eventFlags, UInt64* eventIds) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 356
at System.IO.FileSystemWatcher.RunningInstance.StaticWatcherRunLoopManager.WatchForFileSystemEventsThreadStart(ManualResetEventSlim runLoopStarted, SafeEventStreamHandle eventStream) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 257
at System.IO.FileSystemWatcher.RunningInstance.StaticWatcherRunLoopManager.<>c.b__3_0(Object args) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 210
at System.Threading.Thread.StartCallback() in //src/mono/netcore/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 287
[ERROR] FATAL UNHANDLED EXCEPTION: System.NullReferenceException: Object reference not set to an instance of an object
at System.IO.FileSystemWatcher.RunningInstance.FileSystemEventCallback(IntPtr streamRef, IntPtr clientCallBackInfo, IntPtr numEvents, Byte** eventPaths, FSEventStreamEventFlags* eventFlags, UInt64* eventIds) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 356
at System.Xml.Linq.ElementWriter.GetPrefixOfNamespace(XNamespace ns, Boolean allowDefaultNamespace) in //src/libraries/System.Private.Xml.Linq/src/System/Xml/Linq/XLinq.cs:line 305
at System.IO.FileSystemWatcher.RunningInstance.StaticWatcherRunLoopManager.WatchForFileSystemEventsThreadStart(ManualResetEventSlim runLoopStarted, SafeEventStreamHandle eventStream) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 257
at System.IO.FileSystemWatcher.RunningInstance.StaticWatcherRunLoopManager.<>c.b__3_0(Object args) in //src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.OSX.cs:line 210
at System.Threading.Thread.StartCallback() in /_/src/mono/netcore/System.Private.CoreLib/src/System/Threading/Thread.Mono.cs:line 287
/private/tmp/helix/working/B8CD09E2/w/B3A3097C/e
----- end Mon Jun 8 16:13:32 PDT 2020 ----- exit code 1 -------------------------------------

[ericstj]

Dup of #30056