[INTERNAL] fixed NPE in WhiteboardManager

Happened when closing a session without inviting someone.

Change-Id: I47beebb6b19ebbaf509baa838f5d83fd67cad807
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1763
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>