fix missing null check

bramble-core/src/main/java/org/briarproject/bramble/mailbox/MailboxServiceImpl.java

@@ -4,6 +4,7 @@ import org.briarproject.bramble.api.contact.Contact;
 import org.briarproject.bramble.api.contact.ContactId;
 import org.briarproject.bramble.api.contact.ContactManager;
 import org.briarproject.bramble.api.contact.ContactType;
+import org.briarproject.bramble.api.contact.PrivateMailbox;
 import org.briarproject.bramble.api.contact.event.ContactAddedEvent;
 import org.briarproject.bramble.api.db.DatabaseComponent;
 import org.briarproject.bramble.api.db.DbException;
@@ -95,7 +96,9 @@ public class MailboxServiceImpl implements MailboxService, EventListener {
 			LOG.info("Starting Mailbox Service");
 
 		try {
-			privateMailboxId = contactManager.getPrivateMailbox().getId();
+			PrivateMailbox mb = contactManager.getPrivateMailbox();
+			if (mb!=null)
+				privateMailboxId = mb.getId();
 		} catch (DbException e1) {
 			if (LOG.isLoggable(WARNING))
 				LOG.info(e1.toString());
@@ -129,7 +132,9 @@ public class MailboxServiceImpl implements MailboxService, EventListener {
 			if (hasPrivateMailbox.get())
 				return;
 			try {
-				privateMailboxId = contactManager.getPrivateMailbox().getId();
+				PrivateMailbox mb = contactManager.getPrivateMailbox();
+				if (mb!=null)
+					privateMailboxId = mb.getId();
 			} catch (DbException e1) {
 				if (LOG.isLoggable(WARNING))
 					LOG.info(e1.toString());