title: How to solve Flutter error Column's children must not contain any null values, but a null value was found at index 0?
body: <p>I was learning and trying to create one app using one stateful widget to display a list. My code looks like the following:</p>
  
  <p><strong>main.dart</strong></p>
  
  <pre><code>import 'package:flutter/material.dart';
  import './widgets/user_transactions.dart';
  
  void main() {
    runApp(MyApp());
  }
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return MaterialApp(
        home: Scaffold(
          body: Container(
            child: UserTransactions(),
          ),
        ),
      );
    }
  }
  </code></pre>
  
  <p>and the stateul widget code is: 
  <strong>user_transactions.dart</strong></p>
  
  <pre><code>import 'package:flutter/material.dart';
  import '../models/transaction.dart';
  
  class UserTransactions extends StatefulWidget {
    @override
    _UserTransactionsState createState() =&gt; _UserTransactionsState();
  }
  
  class _UserTransactionsState extends State&lt;UserTransactions&gt; {
    final List&lt;Transaction&gt; _userTransactionLists = [
      Transaction(name: 'boss 01'),
      Transaction(name: 'boss 02'),
      Transaction(name: 'boss 03'),
      Transaction(name: 'boss 04'),
    ];
    @override
    Widget build(BuildContext context) {
      print('==========================================');
      print(_userTransactionLists.length);
      return Column(
        children: _userTransactionLists.map((tx) {
          Text(tx.name);
          print(tx.name);
        }).toList(),
      );
    }
  }
  </code></pre>
  
  <p>And the transaction class looks like this:</p>
  
  <p><strong>Transaction.dart</strong></p>
  
  <pre><code>import 'package:flutter/foundation.dart';
  
  class Transaction {
    final String name;
    Transaction({@required this.name});
  }
  </code></pre>
  
  <p>But getting the error:</p>
  
  <p>══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY
  ╞═══════════════════════════════════════════════════════════
  <strong>The following assertion was thrown building UserTransactions(dirty, state:
  _UserTransactionsState#c1fe3):
  Column's children must not contain any null values, but a null value was
  found at index 0
  The relevant error-causing widget was:
  UserTransactions org-dartlang-app:///packages/test_01/main.dart:14:18</strong></p>
  
  <p>I tried long time but still could not figure out. And when I was tying to debug, I am geeing the correct output using print line. It looks like the following:</p>
  
  <p><a href="https://i.stack.imgur.com/ycRKz.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ycRKz.png" alt="enter image description here"></a></p>