[cleanup] Avoid potential NPE on SessionEventBroker getting

- Avoid potential NPE on SessionEventBroker getting when session is
closed and sequence diagram editor not yet closed.

Change-Id: I2f7c54fb118cfafddeb40884408acfb24f5f1d4a
Signed-off-by: Esteban Dugueperoux <esteban.dugueperoux@obeo.fr>
1 file changed