getting this error showing in red screen I update my packages _TypeError Another exception was thrown: type 'Null' is not a subtype of type 'String' I tried to update the sdk and I'm not understanding where the problem is in this code. I added CircularProgressIndicator with condition statements The relevant error-causing widget was: quizpage quizpage: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',
),
),
),
),
),
],
),
),
);
}
}
CodePudding user response：
In your code some place your are show some variable in Text widget that might be null so change them to these:
Text(
mydata[1][i.toString()][k] ?? "",//<--- add this
style: TextStyle(
color: Colors.white,
fontFamily: "Alike",
fontSize: 16.0,
),
maxLines: 1,
)
and
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",
),
),
),
),
CodePudding user response：
Can you check if your expression is evaluating to null in Text() Widget, just put some constant strings in text widgets and try to run. if expression is evaluating to null, use ?? operation to provide default string value to Text Widget.