Fix error when closing sockets

This occurs when ZeroNet is started and you send a SIGINT signal (for example with CTRL+C).

src/Ui/UiServer.py

@@ -181,7 +181,8 @@ def stop(self):
             from Debug import DebugReloader
             DebugReloader.watcher.stop()
 
-        self.server.socket.close()
+        for socket in self.server.sockets:
+            socket.stop()
         self.server.stop()
         self.running = False
         time.sleep(1)