Uncaught TypeError: Cannot read property 'clear' of null - when one user is leaving the editing session

[oster]

When a user is leaving the editing session, the following exception is triggered at the other site:

Uncaught TypeError: Cannot read property 'clear' of null
   at e._next (cursor.service.ts:42)
   at e.__tryOrUnsub (Subscriber.js:223)
   at e.next (Subscriber.js:172)
   at e._next (Subscriber.js:125)
   at e.next (Subscriber.js:89)
   at e.next (Subject.js:55)
   at e.next (ReplaySubject.js:28)
   at e._next (collaborators.service.ts:39)
   at e.__tryOrUnsub (Subscriber.js:223)
   at e.next (Subscriber.js:172)