Recurring event generate interface error

Steps to reproduce:

• Create a recurring event: 19/11/2019 09:00 - 09:30, recur start on 19/11, monthly on 18th day of month, until date 19/11/2019.
  (focus is on until date that is equal to event start)

Current behaviour:

• An interface error is shown and a log entry is reported (see below)

Expected behaviour:

• No error is displayed

 
2019-11-15 15:10:22 [ERROR] com.sonicle.webtop.calendar.Service - Error in ManageEventsScheduler
java.lang.NullPointerException: null
        at org.joda.time.base.AbstractPartial.compareTo(AbstractPartial.java:314)
        at org.joda.time.LocalDate.compareTo(LocalDate.java:687)
        at com.sonicle.webtop.core.util.ICal4jUtils.calculateRecurrenceSet(ICal4jUtils.java:568)
        at com.sonicle.webtop.calendar.CalendarManager.calculateRecurringInstances(CalendarManager.java:2761)
        at com.sonicle.webtop.calendar.CalendarManager.listEventInstances(CalendarManager.java:1078)
        at com.sonicle.webtop.calendar.CalendarManager.listEventInstances(CalendarManager.java:1042)
        at com.sonicle.webtop.calendar.Service.processManageEventsScheduler(Service.java:692)