Do not send title changed event if the source of the event is null.
This case can occur in multi-threaded scenario where one thread calls
disposing while some other thread calls impl_sendTitleChangedEvent().

Change-Id: I86a65824ab5c60a3fd8d3fb323f6d78944b57a23
Reviewed-on: https://gerrit.libreoffice.org/23736Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>