Possible NPE in TaskEndedHistoryJsonTransformer.java in CMMN engine

**Describe the bug**
NPE possible given static code analysis.

**Expected behavior**
Don't generate NPE's :)

**Code**
This code [(see here)](https://github.com/flowable/flowable-engine/blob/9926fb6f2ca64e657f6b6c9b40cfa4789f0820de/modules/flowable-cmmn-engine/src/main/java/org/flowable/cmmn/engine/impl/history/async/json/transformer/TaskEndedHistoryJsonTransformer.java#L43):

   @Override

    public void transformJson(HistoryJobEntity job, ObjectNode historicalData, CommandContext commandContext) {

        HistoricTaskInstanceEntity historicTaskInstance = getHistoricTaskEntity(historicalData, commandContext);

        

        if (historicTaskInstance != null) {                                      [** 1 **]

            // The end time is the last update time

            Date lastUpdateTime = getDateFromJson(historicalData, CmmnAsyncHistoryConstants.FIELD_END_TIME);

            if (historicTaskInstance.getLastUpdateTime() == null || !historicTaskInstance.getLastUpdateTime().after(lastUpdateTime)) {

                historicTaskInstance.setLastUpdateTime(lastUpdateTime);

                copyCommonHistoricTaskInstanceFields(historicalData, historicTaskInstance);

            }

            setEndProperties(historicalData, historicTaskInstance);

            

        } else {

            HistoricTaskService historicTaskService = CommandContextUtil.getHistoricTaskService(commandContext);

            HistoricTaskInstanceEntity historicTaskInstanceEntity = historicTaskService.createHistoricTask();

            copyCommonHistoricTaskInstanceFields(historicalData, historicTaskInstanceEntity);

            setEndProperties(historicalData, historicTaskInstance);               [** 2 **]

            historicTaskService.insertHistoricTask(historicTaskInstanceEntity, false);

        }

    }


    protected void setEndProperties(ObjectNode historicalData, HistoricTaskInstanceEntity historicTaskInstance) {

        Date endTime = getDateFromJson(historicalData, CmmnAsyncHistoryConstants.FIELD_END_TIME);

        historicTaskInstance.setEndTime(endTime);                               [** 3 **]

        historicTaskInstance.setDeleteReason(getStringFromJson(historicalData, CmmnAsyncHistoryConstants.FIELD_DELETE_REASON));

   

        Date startTime = historicTaskInstance.getStartTime();

        if (startTime != null && endTime != null) {

            historicTaskInstance.setDurationInMillis(endTime.getTime() - startTime.getTime());

        }

    }

If `historicTaskInstance` is null (see ** 1 **), then the code falls into the `else` block and at ** 2 ** a call is made to `setEndProperties()`. This will cause a NPE when `setEndTime()` is called; see ** 3 **.

Sorry but I am not exactly sure of the fix.

If it is any consolation the code has been like this since it was originally written in May, 2018.