[Fix] Clear after calling the EndPointManager.RemoveListener method

websocket-sharp/Net/HttpListenerPrefixCollection.cs

@@ -166,12 +166,10 @@ namespace WebSocketSharp.Net
       if (_listener.IsDisposed)
         throw new ObjectDisposedException (_listener.GetType ().ToString ());
 
-      _prefixes.Clear ();
+      if (_listener.IsListening)
+        EndPointManager.RemoveListener (_listener);
 
-      if (!_listener.IsListening)
+      _prefixes.Clear ();
-        return;
-
-      EndPointManager.RemoveListener (_listener);
     }
 
     /// <summary>