Fix possible NullReferenceException when a BSMLAutomaticViewController is disabled

Author: nicoco007

BeatSaberMarkupLanguage/ViewControllers/WatcherGroup.cs

@@ -188,6 +188,11 @@ private void CreateWatcher()
 
         private void DestroyWatcher()
         {
+            if (Watcher == null)
+            {
+                return;
+            }
+
 #if HRVC_DEBUG
             Logger.Log.Debug($"Destroying FileSystemWatcher for {ContentDirectory}");
 #endif