check timer for null before attempting to dispose

MediaBrowser.Server.Implementations/IO/DirectoryWatchers.cs

@@ -391,9 +391,12 @@ namespace MediaBrowser.Server.Implementations.IO
 
                 Logger.Info("Timer stopped.");
 
+                if (_updateTimer != null)
+                {
                     _updateTimer.Dispose();
                     _updateTimer = null;
                 }
+            }
 
             var paths = _affectedPaths.Keys.ToList();
             _affectedPaths.Clear();