我试着用biGram实现全文搜索框，我写了下面的代码;

class SearchScreen extends StatefulWidget {
  const SearchScreen({Key? key}) : super(key: key);

  @override
  State<SearchScreen> createState() => _SearchScreenState();
}

class _SearchScreenState extends State<SearchScreen> {
  final TextEditingController searchController = TextEditingController();
  bool isShowUsers = false;
  Map? searchWords;
  bool isLoading = false;

  @override
  void initState() {
    super.initState();
    nGram();
  }

  nGram() async {
    setState(() {
      isLoading = true;
    });
    try {
      Map searchWords = tokenToSearch(searchTerm: searchController.text);
      var query = FirebaseFirestore.instance.collection('posts').limit(30);

      searchWords.forEach((word, bool) {
        query = FirebaseFirestore.instance
            .collection('posts')
            .where("token.${word}", isEqualTo: true);
      });

      var snap = await query.get();

      setState(() {});
    } catch (e) {
      showSnackBar(
        context,
        e.toString(),
      );
    }
    setState(() {
      isLoading = false;
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Form(
          child: TextFormField(
            controller: searchController,
            decoration:
                const InputDecoration(labelText: 'Search for a description...'),
            onFieldSubmitted: (String _) {
              setState(() {
                isShowUsers = true;
              });
              print(_);
            },
          ),
        ),
      ),
      body: isShowUsers
          ? FutureBuilder(
              future: nGram().snap,
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return const Center(
                    child: CircularProgressIndicator(),
                  );
                }
                return ListView.builder(
                  itemCount: (snapshot.data! as dynamic).docs.length,
                  itemBuilder: (context, index) {
                    return ListTile(
                      title: InkWell(
                        child: Text(
                          (snapshot.data! as dynamic).docs[index]
                              ['description'],
                        ),
                      ),
                    );
                  },
                );
              },
            )
          : FutureBuilder(
              future: FirebaseFirestore.instance
                  .collection('posts')
                  .orderBy('datePublished')
                  .get(),
              builder: (context, snapshot) {
                if (!snapshot.hasData) {
                  return const Center(
                    child: CircularProgressIndicator(),
                  );
                }

                return Container();
              },
            ),
    );
  }
}

发生下列错误;
类别'Future'没有执行严修getter 'snap'。接收端：'Future '的示例尝试调用：弹响
请帮助我。我想知道如何纠正代码。
引用：要搜索的令牌

Map<String,dynamic> tokenToSearch({ required String searchTerm }) {
  final int nGramIndex = 2;
  final int length = searchTerm.length;
  Map<String,dynamic> tokenToSearch = {};
  if (length < nGramIndex) {
    tokenToSearch[searchTerm] = true;
  } else {
    int termIndex = 0;
    for (int i = 0; i < length - nGramIndex + 1; i++) {
      final String key = searchTerm.substring(termIndex,termIndex + nGramIndex);
      tokenToSearch[key] = true;
      termIndex++;
    }
  }
  return tokenToSearch;
}