RegexStateMatcher checkSpaceSubset(Set<String> spaces) {
            Objects.requireNonNull(spaces, "spaces must be non-null");
            this.checkSpaceSubset = spaces;
            return this;
        }
Return null if met, why not if it is not met.
clustercontroller-core/src/test/java/com/yahoo/vespa/clustercontroller/core/testutils/WaitCondition.java