[release/5.0] Fix NRE introduced in FileSystemWatcher during nullabil…

…ity annotations (#41339)

* Fix NRE introduced during nullability annotations

* Apply Jeff's suggestions from code review

Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>

Co-authored-by: Krzysztof Wicher <kwicher@microsoft.com>
Co-authored-by: Krzysztof Wicher <mordotymoja@gmail.com>
Co-authored-by: Jeff Handley <jeffhandley@users.noreply.github.com>

src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.Linux.cs

@@ -390,7 +390,12 @@ private void AddDirectoryWatchUnlocked(WatchedDirectory? parent, string director
                     // of the world, but there's little that can be done about that.)
                     if (directoryEntry.Parent != parent)
                     {
-                        directoryEntry.Parent?.Children!.Remove (directoryEntry);
+                        // Work around https://github.com/dotnet/csharplang/issues/3393 preventing Parent?.Children!. from behaving as expected
+                        if (directoryEntry.Parent != null)
+                        {
+                            directoryEntry.Parent.Children!.Remove(directoryEntry);
+                        }
+
                         directoryEntry.Parent = parent;
                         if (parent != null)
                         {
@@ -443,7 +448,12 @@ private void RemoveWatchedDirectory(WatchedDirectory directoryEntry, bool remove
                 Debug.Assert (_includeSubdirectories);
                 lock (SyncObj)
                 {
-                    directoryEntry.Parent?.Children!.Remove(directoryEntry);
+                    // Work around https://github.com/dotnet/csharplang/issues/3393 preventing Parent?.Children!. from behaving as expected
+                    if (directoryEntry.Parent != null)
+                    {
+                        directoryEntry.Parent.Children!.Remove(directoryEntry);
+                    }
+
                     RemoveWatchedDirectoryUnlocked (directoryEntry, removeInotify);
                 }
             }