A person in my contact list (using Facebook or Gtalk jabber protocol) initiates a chat session with me.  A conversation box appears at the bottom of the screen, in which the conversation can be held.   
Closing this box (with the close button) leads to a crash of the notification bar.   Pressing the <Super> key brings up the Activities overview, but the notification bar is missing from it.
Conversation can be continued by opening the telepathy client.   

Restarting Gnome-shell (with Alt-F2 ~ r) brings back the notification bar and the conversation dialog box at the bottom of the screen.  Closing this box again reproduces the problem.