Environment

• Team Calendars 6.0.x

Steps to reproduce

1. Create a calendar;
2. Add an event with a participant (in the Who field);
3. Export the calendar to .ics;
4. Import the .ics file in another instance.

Expected behavior

The calendar is imported with no errors.

Actual behavior

NullPointerException appears in the importing window. Confluence logs show the following:

2018-07-12 16:52:01,080 ERROR [http-nio-8090-exec-2] [extra.calendar3.xwork.ImportSubCalendarAction] execute Unable to import events of type other
 -- referer: https://confluence.test.com/display/TEST/calendars | url: /plugins/calendar/importsubcalendar.action | traceId: 2902141a06b26b09 | userName: admin | action: importsubcalendar
java.lang.NullPointerException
	at com.atlassian.confluence.extra.calendar3.DefaultActiveObjectsServiceWrapper.createInviteeEntity(DefaultActiveObjectsServiceWrapper.java:159)
	at com.atlassian.confluence.extra.calendar3.calendarstore.AbstractCalendarDataStore.setSubCalendarContent(AbstractCalendarDataStore.java:1004)
	at com.atlassian.confluence.extra.calendar3.calendarstore.AbstractPeopleHandlingSubCalendarDataStore.setSubCalendarContent(AbstractPeopleHandlingSubCalendarDataStore.java:111)

In spite of that, all the events are imported correctly with no participants - which is expected since the related user identifier (user_key) doesn't exist in the new instance.

Additional details

This bug seems to be caused by a user checker introduced in TC 6 to fix CONFSERVER-55887

Workaround

1. Uninstall Team Calendars 6.0.x in the Confluence instance
2. Download Team Calendars version 5.3.20
3. Navigate to the Manage apps Page
4. Select Upload Apps
5. Choose to upload the team-calendars-5.3.20.obr file that was just downloaded in step 2
6. Create a new Team Calendar
7. Import the ICS file into the newly created Team Calendar
8. Once the import is successful, Update the Team calendar to TC 6.0.x (the latest version)