不能将forEach用于async操作，因为它的回调是VoidCallback而不是AsyncCallback，因此无法返回任何值。
有效Dart建议使用for循环代替：

for (final element in item.bids) {
  final bid = await firestoreRepository.getBidByBidId(bidID: element);
}

尝试Future for循环，如下所示：

await Future.forEach(item.bids, (element) async{
      Bid? bid = await firestoreRepository.getBidByBidId(bidID: element);
      if (bid != null) {
        DbUser? dbUser = await firestoreRepository.getDBUserByDBUserId(
            dbUserID: bid.bidderID);
        if (dbUser != null) {
          bid.userName = dbUser.userName;
          bids.add(bid);
        }
      }
    });

试试这个：

class BidBloc extends Bloc<BidEvent, BidState> {
  final FirestoreRepository firestoreRepository;

  BidBloc({required this.firestoreRepository}) : super(BidsLoadingState()) {

    on<LoadAllBidsEvent>(_LoadBirdEvent);
  }
  
  _LoadBirdEvent(
      LoadAllBidsEvent event,
      Emitter<BidState> emit,
      ) async {
    emit(BidsLoadingState());
    Item item = event.item;

    List<Bid> bids = await getBids();

    bids.sort((a, b) => a.timestamp.compareTo(b.timestamp));
    
    BidsLoadedState(bids);
  }

  Future getBids() async {
    List<Bid> bids = [];

    item.bids?.forEach((element) async {
      Bid? bid = await firestoreRepository.getBidByBidId(bidID: element);
      if (bid != null) {
        DbUser? dbUser = await firestoreRepository.getDBUserByDBUserId(
            dbUserID: bid.bidderID);
        if (dbUser != null) {
          bid.userName = dbUser.userName;
          bids.add(bid);
        }
      }
    });
    return bids;
  }
}

我想这就是你要找的。具体来说，这篇文章中Irl的答案dart await on constructor