An unexpected null key in HashSet, the first be added not the rest

I'm learning Activiti 7, I drew a BPMN diagram as below: When the highlight1 UserTask has been completed but the highlight2 UserTask is still pending, I ran the following code to highlight the completed flow element.

private AjaxResponse highlightHistoricProcess(@RequestParam("instanceId") String instanceId,
      @AuthenticationPrincipal UserInfo userInfo) {
    try {
      // Get the instance from the history table
      HistoricProcessInstance instance = historyService
          .createHistoricProcessInstanceQuery().processInstanceId(instanceId).singleResult();
      BpmnModel bpmnModel = repositoryService.getBpmnModel(instance.getProcessDefinitionId());
      Process process = bpmnModel.getProcesses().get(0);
      // Get all process elements, including sequences, events, activities, etc.
      Collection<FlowElement> flowElements = process.getFlowElements();
      Map<String, String> sequenceFlowMap = Maps.newHashMap();
      flowElements.forEach(e -> {
        
        if (e instanceof SequenceFlow) {
          SequenceFlow sequenceFlow = (SequenceFlow) e;
          String sourceRef = sequenceFlow.getSourceRef();
          String targetRef = sequenceFlow.getTargetRef();
          sequenceFlowMap.put(sourceRef + targetRef, sequenceFlow.getId());
        }
      });

      // Get all historical Activities, i.e. those that have been executed and those that are currently being executed
      List<HistoricActivityInstance> actList = historyService.createHistoricActivityInstanceQuery()
          .processInstanceId(instanceId)
          .list();

      // Each history Activity is combined two by two
      Set<String> actPairSet = new HashSet<>();
      for (HistoricActivityInstance actA : actList) {
        for (HistoricActivityInstance actB : actList) {
          if (actA != actB) {
            actPairSet.add(actA.getActivityId() + actB.getActivityId());
          }
        }
      }

      // Highlight Link ID 
      Set<String> highSequenceSet = Sets.newHashSet();
      actPairSet.forEach(actPair -> {
        logger.info("actPair:{}, seq:{}", actPair, sequenceFlowMap.get(actPair));
        highSequenceSet.add(sequenceFlowMap.get(actPair));
        logger.info("{}",highSequenceSet.toString());
      });

      // Get the completed Activity
      List<HistoricActivityInstance> finishedActList = historyService
          .createHistoricActivityInstanceQuery()
          .processInstanceId(instanceId)
          .finished()
          .list();

      // Highlight the completed Activity
      Set<String> highActSet = Sets.newHashSet();
      finishedActList.forEach(point -> highActSet.add(point.getActivityId()));

      // Get the pending highlighted node, i.e. the currently executing node
      List<HistoricActivityInstance> unfinishedActList = historyService
          .createHistoricActivityInstanceQuery()
          .processInstanceId(instanceId)
          .unfinished()
          .list();
      Set<String> unfinishedPointSet = Sets.newHashSet();
      unfinishedActList.forEach(point -> unfinishedPointSet.add(point.getActivityId()));

      ...
      return AjaxResponse.ajax(ResponseCode.SUCCESS.getCode(),
          ResponseCode.SUCCESS.getDesc(),
          null);
    } catch (Exception e) {
      e.printStackTrace();
      return AjaxResponse.ajax(ResponseCode.ERROR.getCode(),
          "highlight failure",
          e.toString());
    }
  }

Please see this piece of code:

      // Highlight Link ID 
      Set<String> highSequenceSet = Sets.newHashSet();
      actPairSet.forEach(actPair -> {
        logger.info("actPair:{}, seq:{}", actPair, sequenceFlowMap.get(actPair));
        highSequenceSet.add(sequenceFlowMap.get(actPair));
        logger.info("{}",highSequenceSet.toString());
      });

It was expected to get 2 elements in the highSequenceSet, but it got 3, with a unexpected null. The log printed in the console was:

Why is the first null added to the HashSet but not the rest?

Why is the first null added to the HashSet but not the rest?

HashSet implements the Set interface, duplicate values are not allowed.