should never happen in the code path, e.g.,
return the current FeedItem in ItemFragment UI.
Mark underlying method as @Nullable, and let observer's existing
onError() to handle null case
Rx observer : underlying sources marked as @Nullable
ItemFragment.load : .loadInBackground
MediaplayerActivity.checkFavorite : DBReader.getFeedItem
ItemDescriptionFragment.onViewCreated : DBReader.getFeedItem
ItemlistFragment.loadItems : .loadData
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