IllegalArgumentException when adding member to private group
* Android version: 6.0.1 * Phone model: Neffos Y5L (TP801A) * Briar version: 1.1.1 (9476782) * User feedback: "Crash while trying to add person to a group." Stacktrace: ``` java.lang.IllegalArgumentException at org.briarproject.briar.privategroup.invitation.SessionParserImpl.parseCreatorSession(SessionParserImpl.java:54) at org.briarproject.briar.privategroup.invitation.GroupInvitationManagerImpl.isInvitationAllowed(GroupInvitationManagerImpl.java:482) at org.briarproject.briar.android.privategroup.creation.CreateGroupControllerImpl.isDisabled(CreateGroupControllerImpl.java:121) at org.briarproject.briar.android.contactselection.ContactSelectorControllerImpl.lambda$loadContacts$0$ContactSelectorControllerImpl(ContactSelectorControllerImpl.java:51) at org.briarproject.briar.android.contactselection.ContactSelectorControllerImpl$$Lambda$0.run(Unknown Source) at org.briarproject.briar.android.controller.DbControllerImpl.lambda$runOnDbThread$0$DbControllerImpl(DbControllerImpl.java:35) at org.briarproject.briar.android.controller.DbControllerImpl$$Lambda$0.run(Unknown Source) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) at java.lang.Thread.run(Thread.java:818) ```
issue