Steps to Reproduce

run the attached example calendar-event-updates.zul / CalendarTestComposer.java

case 1: click button "short to long event"
case 2: click button "long to longer event"

both cases update the endDate of a CalendarEvent

Current Result

case 1: JS error

Uncaught (in promise) TypeError: Cannot read property 'getTime' of null
    at _compareStartTime (calendar.wpd:69)
    at dateSorting_ (calendar.wpd:530)
    at Array.sort (<anonymous>)
    at init._rePositionDay (calendar.wpd:3077)
    at init.reAlignEvents_ (calendar.wpd:3328)
    at init.setModifyDayEvent (calendar.wpd:509)
    at Function._set2 (zk.wpd:12126)
    at init.setter.func (zk.wpd:21729)
    at init.zk.Widget.zk.$extends.set (zk.wpd:21757)
    at Object.setAttr (zk.wpd:30296)

the "short event" div is rendered below the calendar control

case 2:
the updated "long event" doesn't fully render, it doesn't extend into the second week

Expected Result

after switching to the default mold and back into the month mold the correct result is rendered

Debug Information

Workaround

call calendarModel.remove(item)/add(item) instead of calendarModel.update(item)