Exception on set debugPaintSizeEnabled = true #17244

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart' show debugPaintSizeEnabled;

void main() {
//  debugPaintSizeEnabled = true; 
  runApp(new FriendlychatApp());
}

const String _name = "Your Name";

class FriendlychatApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: "Friendlychat", 
      home: new ChatScreen(),
    );
  }
}

class ChatMessage extends StatelessWidget {
  ChatMessage({this.text, this.animationController});
  final String text;
  final AnimationController animationController;
  @override
  Widget build(BuildContext context) {
      return new SizeTransition(
        sizeFactor: new CurvedAnimation(
          parent: animationController,
          curve: Curves.easeOut
        ),
        axisAlignment: 0.0,
        child: new Container(
          margin: const EdgeInsets.symmetric(vertical: 10.0),
          child: new Row(
            crossAxisAlignment: CrossAxisAlignment.start,
            children: <Widget>[
              new Container(
                margin: const EdgeInsets.only(right: 16.0),
                child: new CircleAvatar(child: new Text(_name[0])),
              ),
              new Expanded(
                child: new Column(
                  crossAxisAlignment: CrossAxisAlignment.start,
                  children: <Widget>[
                    new Text(_name, style: Theme.of(context).textTheme.subhead),
                    new Container(
                      margin: const EdgeInsets.only(top: 5.0),
                      child: new Text(text),
                    ),
                  ],
                ),
              ),
            ],
          ),
        ) 
      );
  }
}

class ChatScreen extends StatefulWidget {
  @override
  State createState() => new ChatScreenState();
}

class ChatScreenState extends State<ChatScreen> with TickerProviderStateMixin {
  final List<ChatMessage> _messages = <ChatMessage>[];
  final TextEditingController _textController = new TextEditingController();
  bool _isComposing = false;

  void _handleSubmitted(String text) {
    _textController.clear();
    setState(() {
      _isComposing = false;
    });
    ChatMessage message = new ChatMessage(
      text: text,
      animationController: new AnimationController(
        duration: new Duration(milliseconds: 700),
        vsync: this, //TickerProviderStateMixin
      ),
    );
    setState(() {
      _messages.insert(0, message);
    });
    message.animationController.forward();
  }

  void dispose() {
    for (ChatMessage message in _messages)
    message.animationController.dispose();
    super.dispose();
  }

  Widget _buildTextComposer() {
    return new IconTheme(
      data: new IconThemeData(color: Theme.of(context).accentColor),
      child: new  Container(
        margin: const EdgeInsets.symmetric(horizontal: 8.0), //margin left and right
        child: new Row(
          children: <Widget>[
            new Flexible(
              child: new TextField(
                controller: _textController,
                onChanged: (String text) {
                  setState(() {
                    _isComposing = text.length > 0;
                  });
                },
                onSubmitted: _handleSubmitted,
                decoration: new InputDecoration.collapsed(
                  hintText: "Send a message",
                ),
                maxLines: null,
              ) ,
            ),

            new Container(
              margin: new EdgeInsets.symmetric(horizontal: 4.0),
              child: new IconButton(
                icon: new Icon(Icons.send),
                onPressed: _isComposing ? () => _handleSubmitted(_textController.text) : null, //null -> disable button 
              ),
            ),

          ],
        ),         
      ),
    );
  }

  @override 
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Friendly Chat'), //Title at the top of app (just bellow the status bar of phone)
      ),

      body: new Column(
        children: <Widget>[
          new Flexible(
            child: new ListView.builder(
              padding: new EdgeInsets.all(8.0),
              reverse: true,
              itemBuilder: (_, int index) => _messages[index],
              itemCount: _messages.length,
            ),
          ),
          new Divider(height: 1.0,),
          new Container(
            decoration: new BoxDecoration(
              color: Theme.of(context).cardColor,
            ),
            child: _buildTextComposer(),
          ) 
        ],
      ),
    );
  }
}

I/flutter ( 6046): #117    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990:15)
I/flutter ( 6046): #118    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930:9)
I/flutter ( 6046): #119    _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.scheduleWarmUpFrame.<anonymous closure> (package:flutter/src/scheduler/binding.dart:751:7)
I/flutter ( 6046): #121    _Timer._runTimers (dart:isolate/runtime/libtimer_impl.dart:382:19)
I/flutter ( 6046): #122    _Timer._handleMessage (dart:isolate/runtime/libtimer_impl.dart:416:5)
I/flutter ( 6046): #123    _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:165:12)
I/flutter ( 6046): (elided 3 frames from class _AssertionError and package dart:async)
I/flutter ( 6046):
I/flutter ( 6046): The following RenderObject was being processed when the exception was fired:
I/flutter ( 6046):   RenderViewport#80a19
I/flutter ( 6046):   creator: Viewport ← _ScrollableScope ← IgnorePointer-[GlobalKey#97fb9] ← Semantics ← Listener ←
I/flutter ( 6046):   _GestureSemantics ← RawGestureDetector-[LabeledGlobalKey<RawGestureDetectorState>#83270] ←
I/flutter ( 6046):   _ExcludableScrollSemantics-[GlobalKey#fc6db] ← RepaintBoundary ← CustomPaint ← RepaintBoundary ←
I/flutter ( 6046):   NotificationListener<ScrollNotification> ← ⋯
I/flutter ( 6046):   parentData: <none> (can use size)
I/flutter ( 6046):   constraints: BoxConstraints(0.0<=w<=360.0, 0.0<=h<=511.0)
I/flutter ( 6046):   layer: OffsetLayer#a1d9c DETACHED
I/flutter ( 6046):   size: Size(360.0, 511.0)
I/flutter ( 6046):   axisDirection: up
I/flutter ( 6046):   crossAxisDirection: right
I/flutter ( 6046):   offset: ScrollPositionWithSingleContext#47b49(offset: 0.0, range: 0.0..0.0, viewport: 511.0,
I/flutter ( 6046):   ScrollableState, AlwaysScrollableScrollPhysics -> ClampingScrollPhysics, IdleScrollActivity#a5bb0,
I/flutter ( 6046):   ScrollDirection.idle)
I/flutter ( 6046):   anchor: 0.0
I/flutter ( 6046): This RenderObject had the following descendants (showing up to depth 5):
I/flutter ( 6046):   RenderSliverPadding#df3db relayoutBoundary=up1
I/flutter ( 6046):     RenderSliverList#94912 relayoutBoundary=up2 NEEDS-PAINT
I/flutter ( 6046): ════════════════════════════════════════════════════════════════════════════════════════════════════
I/flutter ( 6046): Another exception was thrown: Tried to paint a RenderObject reentrantly.