why I got the error in DateTime _selectedDate;?

Question

this is my code

import 'package:flutter/src/widgets/container.dart';
import 'package:flutter/src/widgets/framework.dart';
import 'package:intl/intl.dart';
import 'package:flutter/material.dart';

class NewTransaction extends StatefulWidget {
  final Function addTx;

  NewTransaction(this.addTx);

  @override
  State<NewTransaction> createState() => _NewTransactionState();
}

class _NewTransactionState extends State<NewTransaction> {
  final _titleController = TextEditingController();
  final _amountController = TextEditingController();
  DateTime _selectedDate;


  void _submitData() {
    final enteredTitle = _titleController.text;
    final enteredAmount = double.parse(_amountController.text);

    if (enteredTitle.isEmpty || enteredAmount <= 0) {
      return;
    }
    widget.addTx(
      enteredTitle,
      enteredAmount,
    );
    Navigator.of(context).pop();
  }

  void _presentDatePicker() {
    showDatePicker(
      context: context,
      initialDate: DateTime.now(),
      firstDate: DateTime(2019),
      lastDate: DateTime.now(),
    ).then((pickedDate) {
      if (pickedDate == null) {
        return;
      }
      setState(() {
        _selectedDate = pickedDate;
      });
    });
    print('...');
  }

  @override
  Widget build(BuildContext context) {
    return Card(
      elevation: 5,
      child: Container(
        padding: EdgeInsets.all(10),
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.end,
          children: <Widget>[
            TextField(
              decoration: InputDecoration(labelText: 'Title'),
              controller: _titleController,
              onSubmitted: (_) => _submitData(),

              // onChanged: (val) {
              //   titleInput = val;
              // },
            ),
            TextField(
              decoration: InputDecoration(labelText: 'Amount'),
              keyboardType: TextInputType.number,
              onSubmitted: (_) => _submitData(),
              controller: _amountController,
              // onChanged: (val) => amountInput = val,
            ),
            Container(
              height: 70,
              child: Row(
                children: <Widget>[
                  Expanded(
                    child: Text(
                      _selectedDate == null
                          ? 'No Date Chosen!'
                          : 'Picked Date: ${DateFormat.yMd().format(_selectedDate)}',
                    ),
                  ),
                  TextButton(
                    child: Text(
                      'Choose Date',
                      style: TextStyle(
                        color: Theme.of(context).primaryColor,
                        fontWeight: FontWeight.bold,
                      ),
                    ),
                    onPressed: _presentDatePicker,
                  )
                ],
              ),
            ),
            ElevatedButton(
              child: Text(
                'Add Transaction',
                style: TextStyle(
                  color: Colors.white,
                ),
              ),
              onPressed: _submitData,
              style: ButtonStyle(
                backgroundColor: MaterialStateProperty.resolveWith(
                  (states) {
                    return Colors.purple;
                  },
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}

there's a redline on DateTime _selectedDate;

what should I do?

Answer 1

You're trying to make it null in the code so that you check over it with e _selectedDate == null.

well, you can tell Dart that it can be null from its declaration with a ?:

  DateTime? _selectedDate; // add ?