Stops after getting service

After
` .then(server => {
log('Getting Battery Service...');
return server.getPrimaryService('battery_service');
})`
It doesn't work and doesn't show any error either.

I changed the UUID to other services and it stops after getting service as well.
However, it shows that the device is connected!
I am sure that I put the UUID of the server in optionalService when i start the request.

One more question is that what the difference of optionalService and Service is. If I use optionalService, it works well but if i use Service, there will be error.

I am stuck for two weeks now... anyone knows what happened? Thanks!