import 'package:flutter/material.dart';

void main() => runApp(MyApp());

/// This Widget is the main application widget.
class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

  @override
  _MyStatefulWidgetState createState() => _MyStatefulWidgetState();
}

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('Sample Code'),
      ),
      body: ListView(
        children: [
          Container(),//<-- Error occurs when scrolling
          for(int i=0; i<30; i++)
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: Text('Hello'),
          )
        ],
      ),
    );
  }
}

════════ Exception caught by rendering library ═════════════════════════════════
The method '-' was called on null.
Receiver: null
Tried calling: -(0.0)
The relevant error-causing widget was
    ListView 
lib\main.dart:31
════════════════════════════════════════════════════════════════════════════════