flutter - Flutter-Nosuchmethoderror调用getter 'length'为null

Question

刷新页面时，此错误首先开始，然后起作用。

Error: The getter 'isNotEmpty' was called on null. Receiver: null Tried calling: isNotEmpty

class TreePage extends StatefulWidget {
  String vp;
  TreePage({this.vp});
  @override
  _TreePageState createState() => _TreePageState();
}


class _TreePageState extends State<TreePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Colors.blue,
        title: Text('Проверка штрафов',
        style: TextStyle(color: Colors.white,),
        ),
        centerTitle: true,
      ),
      //new return Container(),

      body: new FutureBuilder(
        future: getPhoneFinal(widget.vp),

        //future: codePhone(widget.code),
        builder: (context, snapshot){
          List data = snapshot.data;
          Map shtr = getByKeySh(); 
          Map shtrIm = getByKeyIm();
          Map violQnt;
          if(data.isNotEmpty){
            violQnt = getViolationQnt(data);
          }else{
            violQnt = null;
          }
          if(snapshot.hasError){
            print (snapshot.error);
            return Text('Не удалось получить ответ от сервера',
            style: TextStyle(color: Colors.red,
            fontSize: 22.0)
            );
          }
          else if(snapshot.hasData ){     
              return ListView.builder (
              itemCount: snapshot.data.length, //length of snapshot data
              itemBuilder: (BuildContext ctxt, int index) {              
              return Container(              
                  child:Column(
                    children: <Widget>[  
                      (index == 0)?
                        new Container(
                          child:Column(
                        children: <Widget>[ 
                          new Text('Подтвержденные штрафы:  ${violQnt['approved_qnt']}'), 
                        ]
                        ),


                        )
                      : new Container(),


                       new Container(                                                
                          child: Image.asset(
                            'images/${shtrIm[data[index] ['VId']]}',
                            width: 50,
                            height: 50,
                          ),
                      ),

                      new Container(                                                
                          child: Image.asset(                            
                            'images/${(data[index] ['ProcessStatus']==1)? 'sts-ok.png': 'sts-no.png'}',
                            width: 50,
                            height: 50,
                          ),

                      ),
                       new Container(                                                
                          child: new Text(  

                            '${(data[index] ['IsPaid']==1)? 'Да': 'нет'}',

                          ),

                      ),

                      new Container(                                                
                          child: new Text(                           
                          //'Тип штрафа:  ${data[index]['VDescription']}'
                          'Тип штрафа: ${shtr[data[index] ['VId']]}'


                            ),
                      ),
                      new Container(                                                
                          child: new Text(                           
                          'Адрес:  ${data[index]['VLocation']}'                         
                            ),
                      ),
                      new Container(                                                
                          child: new Text(                           
                          'Дата:  ${data[index]['VTime']}'                         
                            ),
                      ),
                    ],
                  ),
              );
            }
        );       
          }else if(!snapshot.hasData){
            return new Center(child: CircularProgressIndicator(),);
          }
        }),
    );
  }

 // getFine(String vp) {}


}



Future<Map> getPhone(String phone,String code) async{
  String url = 'xxx/xxx/$phone/$code';
  http.Response response = await http.get(url);
  //var sts = response.statusCode;
  //log( response.statusCode);
  //  var ar = {
  //    'sts': response.statusCode,
     //'data': json.decode(response.body)
   //};
  return json.decode(response.body);
}

Future<List> getPhoneFinal(String numbCar) async{
  String url = 'xxx/xxx/$numbCar';
  http.Response response = await http.get(url);
  return json.decode(utf8.decode(response.bodyBytes));
}



Map<String, String> getByKeySh() {
  return {
            '1625': 'Проезд на красный сигнал светофора',
            '1302': 'Проезд на красный сигнал светофора',
            '1230': 'Пересечение сплошной линии',
            '1301': 'Движение по встречной полосе',
            //1301 => 'Выезд на полосу встречного движения',
            '1345': 'Выезд за стоп-линию'
  };
}
  Map<String, String> getByKeyIm() {
  return {
            '1625': '1625.png',
            '1302': '1302.png',
            '1230': '1230.png',
            '1301': '1301.png',
            //1301 => '',
            '1345': '1345.png'
  };


}

Map<String, int> getViolationQnt(List data ) {
        var total_qnt = 0;
        var approved_qnt = 0;
        var paid_qnt = 0;
        var process_qnt = 0;

 // if(data.isNotEmpty){

    for(var i=0; i< data.length; i++){
      total_qnt++;
      if(data[i]['IsPaid'] == 1){
        paid_qnt++;
      }
      if(data[i]['IsPaid'] == 0 && data[i]['ProcessStatus'] == 1){
        approved_qnt++;
      }
      if(data[i]['IsPaid'] == 0 && data[i]['ProcessStatus'] != 1){
        process_qnt++;
      }
    }
 // }
  return {
            'total_qnt': total_qnt,
            'approved_qnt': approved_qnt,
            'paid_qnt': paid_qnt,
            'process_qnt': process_qnt,
  };


}

Answer 1

请使用data.length > 0代替data.isNotEmpty()也可以使用snapshot.hasData检查