Bugfix with listen server

Added null check for keepalive timer callback to prevent server
application from crashing when stopping the listen server when client(s)
connect and initiate the keepalive timer

Server/Core/Server.cs

@@ -207,10 +207,13 @@ namespace xServer.Core
         {
             KeepAlive keepAlive = (KeepAlive)state;
 
-            if (_keepAlives.Contains(keepAlive))
+            if (_keepAlives != null)
             {
-                keepAlive.Client.Disconnect();
-                _keepAlives.Remove(keepAlive);
+                if (_keepAlives.Contains(keepAlive))
+                {
+                    keepAlive.Client.Disconnect();
+                    _keepAlives.Remove(keepAlive);
+                }
             }
         }