at:
https://github.com/belaban/JGroups/blob/3.6/src/org/jgroups/protocols/TP.java#L1398

    public Map<String, String> handleProbe(String... keys) {
        Map<String,String> retval=new HashMap<>(keys != null? keys.length : 2);
        for(String key: keys) {

keys is checked for null and on the next line it is de-refferenced...

this code fill fail with NPE if keys is null...

This issue is discovered by findbugs NP_NULL_ON_SOME_PATH check. Highly recommend running findbugs + coverity (free for OS projects).