抛出另一个例外：类型 'Null' 不是类型 'String' 的子类型

Question

在红屏中显示此错误时，我更新了我的包_TypeError引发了另一个异常：类型"Null"不是类型"String"的子类型我试图更新sdk，但我不知道此代码中的问题在哪里。我添加了带有条件语句的CircularProgressIndicator导致错误的相关小部件是：quizpagequizpage：file:///C:/Users/flutt/Downloads/quiz/flutter-quizstar master/lib/quizpage.dart:57:18

class getjson extends StatefulWidget {

String langname;
getjson(this.langname);

@override
State<getjson> createState() => _getjsonState();
}

class _getjsonState extends State<getjson> {
late String assettoload;

// a function
setasset() {
if (widget.langname == "Science") {
assettoload = "assets/Science.json";
} else if (widget.langname == "Maths") {
assettoload = "assets/Maths.json";
} else if (widget.langname == "History") {
assettoload = "assets/History.json";
} 
}

@override
Widget build(BuildContext context) {
setasset();
// and now we return the FutureBuilder to load and decode JSON
return FutureBuilder(
future:
DefaultAssetBundle.of(context).loadString(assettoload, cache: false),
builder: (context, snapshot) {
if (snapshot.hasData) {
List mydata = json.decode(snapshot.data.toString());
if (mydata == null) {
return Scaffold(
body: Center(
child: Text(
"Loading",
),
),
);
} else {
return quizpage(mydata: mydata);
}}
return CircularProgressIndicator();
},
);
}
}

class quizpage extends StatefulWidget {
final List mydata;

quizpage({required this.mydata});
@override
_quizpageState createState() => _quizpageState(mydata);
}

class _quizpageState extends State<quizpage> {
final List mydata;
_quizpageState(this.mydata);

Color colortoshow = Colors.indigoAccent;
Color right = Colors.green;
Color wrong = Colors.red;
int marks = 0;
int i = 1;
bool disableAnswer = false;
int j = 1;
int timer = 30;
String showtimer = "30";
var random_array;

Map<String, Color> btncolor = {
"a": Colors.indigoAccent,
"b": Colors.indigoAccent,
"c": Colors.indigoAccent,
"d": Colors.indigoAccent,
};

bool canceltimer = false;


genrandomarray(){
var distinctIds = [];
var rand = new Random();
for (int i = 0;; ) {
distinctIds.add(rand.nextInt(10));
random_array = distinctIds.toSet().toList();
if(random_array.length < 10){
continue;
}else{
break;
}
}
print(random_array);
}


@override
void initState() {
starttimer();
genrandomarray();
super.initState();
}

@override
void setState(fn) {
if (mounted) {
super.setState(fn);
}
}

void starttimer() async {
const onesec = Duration(seconds: 1);
Timer.periodic(onesec, (Timer t) {
setState(() {
if (timer < 1) {
t.cancel();
nextquestion();
} else if (canceltimer == true) {
t.cancel();
} else {
timer = timer - 1;
}
showtimer = timer.toString();
});
});
}

void nextquestion() {
canceltimer = false;
timer = 30;
setState(() {
if (j < 10) {
i = random_array[j];
j++;
} else {
Navigator.of(context).pushReplacement(MaterialPageRoute(
builder: (context) => resultpage(marks: marks),
));
}
btncolor["a"] = Colors.indigoAccent;
btncolor["b"] = Colors.indigoAccent;
btncolor["c"] = Colors.indigoAccent;
btncolor["d"] = Colors.indigoAccent;
disableAnswer = false;
});
starttimer();
}

void checkanswer(String k) {

// in the previous version this was
// mydata[2]["1"] == mydata[1]["1"][k]
// which i forgot to change
// so nake sure that this is now corrected
if (mydata[2][i.toString()] == mydata[1][i.toString()][k]) {
// just a print sattement to check the correct working
// debugPrint(mydata[2][i.toString()] + " is equal to " + mydata[1][i.toString()][k]);
marks = marks + 5;
// changing the color variable to be green
colortoshow = right;
} else {
// just a print sattement to check the correct working
// debugPrint(mydata[2]["1"] + " is equal to " + mydata[1]["1"][k]);
colortoshow = wrong;
}
setState(() {
btncolor[k] = colortoshow;
canceltimer = true;
disableAnswer = true;
});
// nextquestion();
// changed timer duration to 1 second
Timer(Duration(seconds: 2), nextquestion);
}

Widget choicebutton(String k) {
return Padding(
padding: EdgeInsets.symmetric(
vertical: 10.0,
horizontal: 20.0,
),
child: MaterialButton(
onPressed: () => checkanswer(k),
child: Text(
mydata[1][i.toString()][k],
style: TextStyle(
color: Colors.white,
fontFamily: "Alike",
fontSize: 16.0,
),
maxLines: 1,
),
color: btncolor[k],
splashColor: Colors.indigo[700],
highlightColor: Colors.indigo[700],
minWidth: 200.0,
height: 45.0,
shape:
RoundedRectangleBorder(borderRadius: BorderRadius.circular(20.0)),
),
);
}

@override
Widget build(BuildContext context) {
SystemChrome.setPreferredOrientations(
[DeviceOrientation.portraitDown, DeviceOrientation.portraitUp]);
return WillPopScope(
onWillPop: () async{
return await showDialog(
context: context,
builder: (context) => AlertDialog(
title: Text(
"Welcome",
),
content: Text("You Can't Go Back At This Stage."),
actions: <Widget>[
ElevatedButton(
onPressed: () {
Navigator.of(context).pop();
},
child: Text(
'Ok',
),
)
],
));
},
child: Scaffold(
body: Column(
children: <Widget>[
Expanded(
flex: 3,
child: Container(
padding: EdgeInsets.all(15.0),
alignment: Alignment.bottomLeft,
child: Text(
mydata[0][i.toString()] ,
style: TextStyle(
fontSize: 16.0,
fontFamily: "Quando",
),
),
),
),
Expanded(
flex: 6,
child: AbsorbPointer(
absorbing: disableAnswer,
child: Container(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
choicebutton('a'),
choicebutton('b'),
choicebutton('c'),
choicebutton('d'),
],
),
),
),
),
Expanded(
flex: 1,
child: Container(
alignment: Alignment.topCenter,
child: Center(
child: Text(
showtimer,
style: TextStyle(
fontSize: 35.0,
fontWeight: FontWeight.w700,
fontFamily: 'Times New Roman',
),
),
),
),
),
],
),
),
);
}
}

Answer 1

在代码的某个地方，您的are在Text小部件中显示了一些可能是null的变量，因此将它们更改为：

Text(
mydata[1][i.toString()][k] ?? "",//<--- add this
style: TextStyle(
color: Colors.white,
fontFamily: "Alike",
fontSize: 16.0,
),
maxLines: 1,
)

和

Expanded(
flex: 3,
child: Container(
padding: EdgeInsets.all(15.0),
alignment: Alignment.bottomLeft,
child: Text(
mydata[0][i.toString()] ?? "",//<--- add this
style: TextStyle(
fontSize: 16.0,
fontFamily: "Quando",
),
),
),
),

Answer 2

你能在Text((Widget中检查你的表达式是否为null吗，只需在文本窗口小部件中放入一些常量字符串并尝试运行即可。如果表达式的计算结果为null，请使用操作，为Text Widget提供默认字符串值。