Issue/2596 fixes coredata threading glitch

Fixes #2596

> gets posted in the ReaderTopicService class, from the main thread

App delegate's `handleDefaultAccountChangedNotification:` does this
```
        NSManagedObjectContext *context     = [[ContextManager sharedInstance] newDerivedContext];
        ReaderTopicService *topicService    = [[ReaderTopicService alloc] initWithManagedObjectContext:context];
        [context performBlock:^{
            ReaderTopic *topic              = topicService.currentTopic;
```

Since it's a derived context, `currentTopic` is called from a background thread. If there's a problem fetching the stored topic, it'll call `setCurrentTopic:nil`, which will post `ReaderTopicDidChangeNotification`

I'm not against the assertion, I just feel is adding a way to alert us of the problem instead of preventing it