Check whether first visible message ID is null before storing.

briar-android/src/main/java/org/briarproject/briar/android/threaded/ThreadListControllerImpl.java

@@ -99,13 +99,13 @@ public abstract class ThreadListControllerImpl<G extends NamedGroup, I extends T
 
 	@Override
 	public void onActivityDestroy() {
+		final MessageId first = listener.getFirstVisibleMessageId();
+		if (first != null) {
 			dbExecutor.execute(new Runnable() {
 				@Override
 				public void run() {
 					try {
-					messageTracker
+						messageTracker.storeMessageId(groupId, first);
-							.storeMessageId(groupId,
-									listener.getFirstVisibleMessageId());
 					} catch (DbException e) {
 						if (LOG.isLoggable(WARNING))
 							LOG.log(WARNING, e.toString(), e);
@@ -113,6 +113,7 @@ public abstract class ThreadListControllerImpl<G extends NamedGroup, I extends T
 				}
 			});
 		}
+	}
 
 	@CallSuper
 	@Override