Flutter : Type mismatch: inferred type is String? but String was expected - flutter

Question

I am new to coding.I am using upi_pay package in my project to make UPI payments getting error as "Type mismatch: inferred type is String? but String was expected" when I tried the build the app
I followed this article https://dev.to/dsc_ciet/adding-upi-payment-gateway-in-flutter-376c
I am new to coding, don't mind if this was a easy thing.
Please go through the below code
Thanks in advance
class PaymentScreen extends StatefulWidget {
#override
_PaymentScreenState createState() => _PaymentScreenState();
}
class _PaymentScreenState extends State<PaymentScreen> {
String _upiAddrError;
final _upiAddressController = TextEditingController();
final _amountController = TextEditingController();
bool _isUpiEditable = false;
Future<List<ApplicationMeta>> _appsFuture;
#override
void initState() {
_amountController.text =
(Random.secure().nextDouble() * 10).toStringAsFixed(2);
_appsFuture = UpiPay.getInstalledUpiApplications();
super.initState();
}
void _generateAmount() {
setState(() {
_amountController.text =
(Random.secure().nextDouble() * 10).toStringAsFixed(2);
});
}
Future<void> _onTap(ApplicationMeta app) async {
final err = _validateUpiAddress(_upiAddressController.text);
if (err != null) {
setState(() {
_upiAddrError = err;
});
return;
}
setState(() {
_upiAddrError = null;
});
final transactionRef = Random.secure().nextInt(1 << 32).toString();
print("Starting transaction with is $transactionRef");
final a = await UpiPay.initiateTransaction(
amount: _amountController.text,
app: app.upiApplication,
receiverName: "Sharad",
receiverUpiAddress: _upiAddressController.text,
transactionRef: transactionRef,
merchantCode: '7372',
);
print(a);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: SafeArea(
child: Container(
padding: EdgeInsets.symmetric(horizontal: 16),
child: ListView(
children: <Widget>[
Container(
margin: EdgeInsets.only(top: 32),
child: Row(
children: <Widget>[
Expanded(
child: TextFormField(
controller: _upiAddressController,
enabled: _isUpiEditable,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: 'address#upi',
labelText: 'Receiving UPI Address',
),
),
),
Container(
margin: EdgeInsets.only(left: 8),
child: IconButton(
icon: Icon(
_isUpiEditable ? Icons.check : Icons.edit,
),
onPressed: () {
setState(() {
_isUpiEditable = !_isUpiEditable;
});
},
),
),
],
),
),
if (_upiAddrError != null)
Container(
margin: EdgeInsets.only(top: 4, left: 12),
child: Text(
_upiAddrError,
style: TextStyle(color: Colors.red),
),
),
Container(
margin: EdgeInsets.only(top: 32),
child: Row(
children: <Widget>[
Expanded(
child: TextField(
controller: _amountController,
readOnly: true,
enabled: false,
decoration: InputDecoration(
border: OutlineInputBorder(),
labelText: 'Amount',
),
),
),
Container(
margin: EdgeInsets.only(left: 8),
child: IconButton(
icon: Icon(Icons.loop),
onPressed: _generateAmount,
),
),
],
),
),
Container(
margin: EdgeInsets.only(top: 128, bottom: 32),
child: Column(
mainAxisSize: MainAxisSize.min,
children: <Widget>[
Container(
margin: EdgeInsets.only(bottom: 12),
child: Text(
'Pay Using',
style: Theme.of(context).textTheme.caption,
),
),
FutureBuilder<List<ApplicationMeta>>(
future: _appsFuture,
builder: (context, snapshot) {
if (snapshot.connectionState != ConnectionState.done) {
return Container();
}
return GridView.count(
crossAxisCount: 2,
shrinkWrap: true,
mainAxisSpacing: 8,
crossAxisSpacing: 8,
childAspectRatio: 1.6,
physics: NeverScrollableScrollPhysics(),
children: snapshot.data
.map((it) => Material(
key: ObjectKey(it.upiApplication),
color: Colors.grey[200],
child: InkWell(
onTap: () => _onTap(it),
child: Column(
mainAxisSize: MainAxisSize.min,
mainAxisAlignment:
MainAxisAlignment.center,
children: <Widget>[
Image.memory(
it.icon,
width: 64,
height: 64,
),
Container(
margin: EdgeInsets.only(top: 4),
child: Text(
it.upiApplication.getAppName(),
),
),
],
),
),
))
.toList(),
);
},
),
],
),
)
],
),
),
),
);
}
}
String _validateUpiAddress(String value) {
if (value.isEmpty) {
return 'UPI Address is required.';
}
if (!UpiPay.checkIfUpiAddressIsValid(value)) {
return 'UPI Address is invalid.';
}
return null;
}

Answer 1

You're assigning null to _upiAddrError but it's a non-nullable String.
Declare that variable as String? _upiAddrError instead of String _upiAddrError to make it nullable.