Steps to replicate:

1. Spin Confluence 6.6.5 and Jira 7.6.4 instances and establish an application link in between them.
2. Install Team Calendars 6.0.14 on Confluence.
3. Add a Calendar.
4. Get to Jira and create a project with a couple issues inside it.
5. On Confluence, add a new event to the Calendar, which is Jira Issues Dates type.
6. Use a JQL on the event that points to that project.
7. Don’t forget to define Start and End date on the issues and use them as dates inside the event.
8. Once done, try to drag and drop the event to change its end date. NPE is thrown. PFA

When NPE is fixed, integration still does not work. Event can be successfully moved, but no communication with Jira happens after that. According to documentation, changes to TC Jira events should be reflected in Jira. This also doesn't work for single issue date.

Most probable this is a regression after refactoring.