I was following the pattern in migration 3:

Without the invariant, we get the following flow errors:

That said, the following would probably be a cleaner approach and avoid use of invariant:

// 2. Get existing `stores` from SQLite.
const stores = await sharedWorker.schedule({
  type: workerRequestMessageTypes.GET_CLIENT_STORE,
});

const messages: ?$ReadOnlyArray<ClientDBMessageInfo> =
  stores?.store?.messages;

if (messages === null || messages === undefined || messages.length === 0) {
  return state;
}

If message.content doesn't exist for a UPDATE_RELATIONSHIP message it won't be removed from localDB, but this situation should not be possible.

Updated to go with the previous check instead to avoid possibility of crashing mid-migration.