setPostAuthenticationChecks in AbstractUserDetailsReactiveAuthenticationManager validates the field instead of the parameter

[big-cir]

Describe the bug

The null check in AbstractUserDetailsReactiveAuthenticationManager#setPostAuthenticationChecks asserts the current field value instead of the parameter:

public void setPostAuthenticationChecks(UserDetailsChecker postAuthenticationChecks) {
    Assert.notNull(this.postAuthenticationChecks, "postAuthenticationChecks cannot be null");
    this.postAuthenticationChecks = postAuthenticationChecks;
}

Since the field is initialized to a default checker, the assertion always passes, so null is silently accepted and the next authenticate(...) call fails with a raw NullPointerException at .doOnNext(this.postAuthenticationChecks::check) instead of failing fast with a clear message.

All other setters in the same class (setPasswordEncoder, setScheduler, setUserDetailsPasswordService, setMessageSource) validate the parameter.

To Reproduce

UserDetailsRepositoryReactiveAuthenticationManager manager =
        new UserDetailsRepositoryReactiveAuthenticationManager(userDetailsService);
manager.setPostAuthenticationChecks(null); // no exception
manager.authenticate(token).block();       // NullPointerException

Expected behavior

setPostAuthenticationChecks(null) should throw IllegalArgumentException("postAuthenticationChecks cannot be null").