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

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;
}

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.