如何在共享首选项中保存Firestore数据，没有为类型'DocumentSnapshot‘定义运算符'[]’

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:cloud_firestore/cloud_firestore.dart';

Future<void> saveFirestoreDataToSharedPreferences(String collectionName, String documentId) async {
  // 获取Firestore数据
  DocumentSnapshot documentSnapshot = await FirebaseFirestore.instance.collection(collectionName).doc(documentId).get();

  if (documentSnapshot.exists) {
    // 将DocumentSnapshot转换为Map
    Map<String, dynamic> data = documentSnapshot.data() as Map<String, dynamic>;

    // 将Map转换为JSON字符串
    String jsonString = jsonEncode(data);

    // 获取SharedPreferences实例
    SharedPreferences prefs = await SharedPreferences.getInstance();

    // 保存JSON字符串到SharedPreferences
    await prefs.setString(documentId, jsonString);
  } else {
    print('Document does not exist');
  }
}

Future<Map<String, dynamic>> getFirestoreDataFromSharedPreferences(String documentId) async {
  // 获取SharedPreferences实例
  SharedPreferences prefs = await SharedPreferences.getInstance();

  // 从SharedPreferences获取JSON字符串
  String jsonString = prefs.getString(documentId);

  if (jsonString != null) {
    // 将JSON字符串转换回Map
    return jsonDecode(jsonString);
  } else {
    return {};
  }
}