SimpMessageHeaderAccessor#getUser is nullable after upgrade to spring-boot-2. How to handle?

Question

Source code under question:

@Configuration
public class PresenceEventListener {    
    @EventListener
    public void handleSessionConnected(SessionConnectEvent event) {
        SimpMessageHeaderAccessor headers = SimpMessageHeaderAccessor.wrap(event.getMessage());
        Principal principal = headers.getUser();
        /* custom logic with principal.getName() */
    }

    @EventListener
    public void handleSessionDisconnect(SessionDisconnectEvent event) {
        SimpMessageHeaderAccessor headers = SimpMessageHeaderAccessor.wrap(event.getMessage());
        Principal principal = headers.getUser();
        /* custom logic with principal.getName() */
    }
}

This code is more or less identical in all examples I could find. Here's one reference...

By upgrading from spring-boot v1.5 to v2, Findbugs complains about .getName() because principal might be null, and is unchecked.

This is because spring-messaging, since v5, has marked method SimpMessageHeaderAccessor#getUser as @Nullable.

My question is how is this supposed to be handled properly? (i.e. when #getUser returns null) Should the code throw an exception? do nothing? And what does it actually mean that the user is null in this context? Could this be considered a hacking attempt ?

Principal principal = headers.getUser();
if (principal == null) {
    // what should happen here ?
}

Thanks !