NullPointerException for ZookeeperAutoServiceRegistration

ZookeeperAutoServiceRegistration throws an exception when ServiceInstanceRegistration method getServiceInstance() is called before ZookeeperAutoServiceRegistration method getPort()

It happens because method getServiceInstance() calls build() and when getPort() will be called it will return null instead 0.
https://github.com/spring-cloud/spring-cloud-zookeeper/blob/ec845a47a00d560e6cf234a342fd31614823a20c/spring-cloud-zookeeper-discovery/src/main/java/org/springframework/cloud/zookeeper/serviceregistry/ServiceInstanceRegistration.java#L64-L69
https://github.com/spring-cloud/spring-cloud-zookeeper/blob/ec845a47a00d560e6cf234a342fd31614823a20c/spring-cloud-zookeeper-discovery/src/main/java/org/springframework/cloud/zookeeper/serviceregistry/ServiceInstanceRegistration.java#L71-L73
https://github.com/spring-cloud/spring-cloud-zookeeper/blob/ec845a47a00d560e6cf234a342fd31614823a20c/spring-cloud-zookeeper-discovery/src/main/java/org/springframework/cloud/zookeeper/serviceregistry/ServiceInstanceRegistration.java#L83-L88
https://github.com/spring-cloud/spring-cloud-zookeeper/blob/ec845a47a00d560e6cf234a342fd31614823a20c/spring-cloud-zookeeper-discovery/src/main/java/org/springframework/cloud/zookeeper/serviceregistry/ZookeeperAutoServiceRegistration.java#L70-L79

Also getInstanceId() method returns null.

Use case when we need to call getServiceInstance for example to create leader candidate and associate with registration id for further understanding which node is a leader.

@Bean
public DefaultCandidate bean(ServiceInstanceRegistration registration) {
    return return new DefaultCandidate(registration.getServiceInstance().getId(), "default");
}

Start by reading ServiceInstanceRegistration.java and ZookeeperAutoServiceRegistration.java at the linked methods, then trace the call order where getServiceInstance() runs before getPort(). Verify the registration can be obtained in that order without a NullPointerException, with a usable port and non-null instance ID.