get method
Implementation
Future<V?> get(String key, [Transaction? txn]) async {
if (_cache.containsKey(key)) return _cache[key];
txn ??= boxCollection._db.transaction(name, 'readonly');
final store = txn.objectStore(name);
_cache[key] = await store.getObject(key) as V?;
return _cache[key];
}