private static void removeAgent(RemoteGfManagerAgent toRemove) {
    synchronized (enumerationSync) {
      List<RemoteGfManagerAgent> replace = new ArrayList<>(allAgents);
      replace.remove(toRemove);
      allAgents = replace;
    }
  }
Is this thread currently sending a message?
geode-core/src/main/java/org/apache/geode/internal/admin/remote/RemoteGfManagerAgent.java