[Modify] Throw exception

websocket-sharp/Server/HttpServer.cs

@@ -630,6 +630,11 @@ namespace WebSocketSharp.Server
     /// </value>
     public ServerSslConfiguration SslConfiguration {
       get {
+        if (!_secure) {
+          var msg = "This instance does not provide secure connections.";
+          throw new InvalidOperationException (msg);
+        }
+
         return _listener.SslConfiguration;
       }
     }