// Modify the _handleSubmittted method definition as follows.

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