我在Flutter dart 代码中收到此错误未处理的异常：类型'Future'不是类型转换中'String'类型的子类型

上下文:我试图使用存储在缓存中的会话ID发送请求.

我能够显示缓存的会话ID，但当我try 使用缓存的会话ID发送请求时，我收到了以下错误:"未处理的异常:类型‘Future’不是类型转换中类型‘String’的子类型"

谢谢你的帮助！

以下是我正在使用的代码:

import 'package:cache_manager/cache_manager.dart';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;
import 'dart:convert';

class messageBoard extends StatefulWidget {
   messageBoard({Key key}) : super(key: key);
   
  @override
  State<messageBoard> createState() => _messageBoardState();
}

class _messageBoardState extends State<messageBoard> {

  final _sessionIDController = ReadCache.getString(key: "cache");

  void messageBoardReq() async {

    var map = <String, dynamic>{
      "SessionID": _sessionIDController,
    };

    var res = await http.post(
      Uri.parse("http://192.168.1.8:8080/HongLeong/MENU_REQUEST.do"),
      body: map,
    );

    final data = jsonDecode(res.body);
    print(data);

  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      body: Column(
        children: [
          Center(
            child: CacheManagerUtils.cacheTextBuilder(
            textStyle: TextStyle(color: Colors.black), cacheKey: "cache"),
          ),
          RawMaterialButton(
            onPressed: () {
              messageBoardReq();
            },
            elevation: 2.0,
            fillColor: Colors.white,
            padding: const EdgeInsets.all(15.0),
            shape: const CircleBorder(),
            child: const Icon(
              Icons.forum,
              size: 35.0,
            ),
          ),
        ],
      )
    );
  }
}