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Question

方法'setStringList'在null上被调用。接收方：null尝试调用：setStringList（“ list”，_GrowableList'的实例（长度：8））。

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我正在使用共享首选项来存储List，方法是使用构造函数的值获取其对象。我得到所有值，但发生以下错误：在null上调用了方法'setStringList'。接收方：null尝试调用：setStringList（“ list”，'_GrowableList'的Instance（length：8））。以下是代码。请尽快解决错误。  我会感激的！

上一页的按下功能仅供参考

    onPressed: () {
                        _showwithdrawSnakbar(context);
                        setState(() {
                        var now = new DateTime.now();
                        var formatter = new DateFormat('dd-MM-yyyy');
                        String formatted = formatter.format(now);
                          if (!stopfromwith) {

                            print("in Withdrawer");
                          var routetorec=MaterialPageRoute(
    builder: (context)=> Transfers(pay:Transactions(formatted,purpose:_currrentpurpose,amount:ctrlwith.text))
                          );

                          print(_currrentpurpose);
                          print(ctrlwith.text);
                          Navigator.of(context).push(routetorec); 

                          }
                                       });
                                              stopfromwith = false;
                                            }

主代码：

import 'dart:convert';

import 'package:flutter/material.dart';
import 'models/Transactions.dart';
import 'package:shared_preferences/shared_preferences.dart';

 List<Transactions> list = List<Transactions> ();


class Transfers extends StatefulWidget {
  Transactions pay;
    Transfers({Key key,this.pay}):super(key:key){
      print("added here now  the item from withdrawer");
      print(pay);
      _addTrasaction(pay);

   print("IN State");
            print(list);
          }

        @override
        _TransfersState createState() => _TransfersState(pay);
      }

 _addTrasaction(Transactions transaction){

   list.add(transaction);
   print("IN _State");
   print(list);//works fine
 }

class _TransfersState extends State<Transfers> {


  Transactions payd;
_TransfersState(this.payd);
  SharedPreferences sharedPreferences;

@override
  void initState() {    
  print(payd);//works fine
   _updatelist();
      initsharedPreferences();
    super.initState();
  }
   initsharedPreferences()async{
    sharedPreferences= await  SharedPreferences.getInstance();
    loadData();
    saveData();
  }
  void saveData(){

    List<String> stringList = list.map(
      (item) => json.encode(item.toMap()
    )).toList();
     // sharedPreferences.remove('List');
      print("it runs ");
      print(stringList);//works fine
// I think the error starts from here
    sharedPreferences.setStringList('list', stringList);

  }
  void loadData(){
  List<String>llist=sharedPreferences.getStringList('list');
  list=llist.map((item)=>Transactions.fromMap(json.decode(item))).toList();
  setState(() { });
}
  _updatelist(){
    setState(() {
    list.add(payd);
    print(list);
      saveData();
    });
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Transfers"),
        centerTitle: true,
        leading: Icon(Icons.repeat),
      ),
      body: list.length == 0
          ? Center(child: Text("No data"))
          : transfersListView(),
    );
  }

  _subCard(Transactions transaction) {
    return Dismissible(
      key: Key(transaction.hashCode.toString()),
      onDismissed: (direction) {
        list.remove(transaction);
      },
      background: Container(

        child: Padding(
          padding: const EdgeInsets.only(left: 168.0),
          child: Icon(
            Icons.delete,
            size: 34,
            color: Colors.white,
          ),
        ),
        color: Colors.red,
      ),
      child: TFCustomCard(transaction.date,transaction.purpose,transaction.amount),
    );
  }

  ListView transfersListView() {
    return ListView.builder(
      itemCount: list.length,
      itemBuilder: (context, index) {
        return _subCard(list[index]);
      },
    );

  }

}



class TFCustomCard extends StatelessWidget {

  String _date,_purpose,_amount;


  TFCustomCard(this._date,this._purpose,this._amount);


  @override
  Widget build(BuildContext context) {
    return Container(/*simple  card with _date,_purpose,and _amount*/);
  }
}