add null check to PeriodicTimer

MediaBrowser.Common/Threading/PeriodicTimer.cs

@@ -14,6 +14,11 @@ namespace MediaBrowser.Common.Threading
 
         public PeriodicTimer(Action<object> callback, object state, TimeSpan dueTime, TimeSpan period)
         {
+            if (callback == null)
+            {
+                throw new ArgumentNullException("callback");
+            }
+
             Callback = callback;
             _period = period;
             _state = state;

MediaBrowser.Server.Implementations/Connect/ConnectEntryPoint.cs

@@ -44,7 +44,7 @@ namespace MediaBrowser.Server.Implementations.Connect
         {
             Task.Run(() => LoadCachedAddress());
 
-            _timer = new PeriodicTimer(null, new TimerCallback(TimerCallback), TimeSpan.FromSeconds(5), TimeSpan.FromHours(3));
+            _timer = new PeriodicTimer(TimerCallback, null, TimeSpan.FromSeconds(5), TimeSpan.FromHours(3));
         }
 
         private readonly string[] _ipLookups = { "http://bot.whatismyipaddress.com", "https://connect.emby.media/service/ip" };