NoSuchMethodError (NoSuchMethodError: The method '[]' was called on null - flutter

Question

I tried to intergrate Dialogflow API in my flutter app ; after I submitted the message I should get a reply from Dialogflow but
I get an error:
Exception has occurred. NoSuchMethodError (NoSuchMethodError: The
method '[]' was called on null.
Receiver: null Tried
calling:)
Here is my code(chatbot.dart):
class Chatbot extends StatefulWidget {
Chatbot({
Key key,
this.title,
}) : super(key: key);
final String title;
#override
_HomePageDialogflowV2 createState() => _HomePageDialogflowV2();
}
class _HomePageDialogflowV2 extends State<Chatbot> {
final List<ChatMessage> _messages = <ChatMessage>[];
final TextEditingController _textController = TextEditingController();
Widget _buildTextComposer() {
return IconTheme(
data: IconThemeData(color: Theme.of(context).accentColor),
child: Container(
margin: const EdgeInsets.symmetric(horizontal: 8.0),
child: Row(
children: <Widget>[
Flexible(
child: TextField(
controller: _textController,
onSubmitted: _handleSubmitted,
decoration:
InputDecoration.collapsed(hintText: "Send a message"),
),
),
Container(
margin: EdgeInsets.symmetric(horizontal: 4.0),
child: IconButton(
icon: Icon(Icons.send),
onPressed: () => _handleSubmitted(_textController.text),
),
),
],
),
),
);
}
void response(query) async {
_textController.clear();
AuthGoogle authGoogle = await AuthGoogle(
fileJson: "assets/credentials-assem.json",
).build();
Dialogflow dialogflow = Dialogflow(
authGoogle: authGoogle,
language: Language.ENGLISH,
);
AIResponse response = await dialogflow.detectIntent(query);
ChatMessage message = ChatMessage(
text: response.getMessage() ??
CardDialogflow(
response.getListMessage()[0],
).title,
name: "Bot",
type: false,
);
setState(() {
_messages.insert(0, message);
});
}
void _handleSubmitted(String text) {
_textController.clear();
ChatMessage message = ChatMessage(
text: text,
name: "Aseem",
type: true,
);
setState(() {
_messages.insert(0, message);
});
response(text);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("ChatBot"),
),
body: Column(children: <Widget>[
Flexible(
child: ListView.builder(
padding: EdgeInsets.all(8.0),
reverse: true,
itemBuilder: (_, int index) => _messages[index],
itemCount: _messages.length,
)),
Divider(height: 1.0),
Container(
decoration: BoxDecoration(color: Theme.of(context).cardColor),
child: _buildTextComposer(),
),
]),
);
}
}
class ChatMessage extends StatelessWidget {
ChatMessage({
this.text,
this.name,
this.type,
});
final String text;
final String name;
final bool type;
List<Widget> otherMessage(context) {
return <Widget>[
Container(
margin: const EdgeInsets.only(right: 16.0),
child: CircleAvatar(
child: Image.asset("images/bot.png"),
),
),
Expanded(
child: Column(
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text(
this.name,
style: TextStyle(fontWeight: FontWeight.bold),
),
Container(
margin: const EdgeInsets.only(top: 5.0),
child: Text(text),
),
],
),
),
];
}
List<Widget> myMessage(context) {
return <Widget>[
Expanded(
child: Column(
crossAxisAlignment: CrossAxisAlignment.end,
children: <Widget>[
Text(this.name, style: Theme.of(context).textTheme.subhead),
Container(
margin: const EdgeInsets.only(top: 5.0),
child: Text(text),
),
],
),
),
Container(
margin: const EdgeInsets.only(left: 16.0),
child: CircleAvatar(
child: Text(this.name[0]),
),
),
];
}
#override
Widget build(BuildContext context) {
return Container(
margin: const EdgeInsets.symmetric(vertical: 10.0),
child: Row(
crossAxisAlignment: CrossAxisAlignment.start,
children: this.type ? myMessage(context) : otherMessage(context),
),
);
}
}
Main.dart:
void main() => runApp(MyApp());
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: 'Flutter Demo Home Page'),
);
}
}
class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);
final String title;
#override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
#override
Widget build(BuildContext context) {
return Chatbot();
}
}
Did I make a mistake somewhere?
Thanks

Answer 1

Go to your Dialogflow agent settings.
Click on the service account link, this will open your Google project service accounts.
Click on your service account --> Add Key --> Create a new key --> JSON and download it.
import the JSON file in your project and insert link it with this statement:
AuthGoogle authGoogle = await AuthGoogle(fileJson: "path-to-your-json-file").build();

Answer 2

I believe this is your problem. void response(query) async should be Future<void> response(query) async and the when you call your response function you need to await it so you have to make the function you call response in async.
void _handleSubmitted(String text) async {
_textController.clear();
ChatMessage message = ChatMessage(
text: text,
name: "Aseem",
type: true,
);
setState(() {
_messages.insert(0, message);
});
await response(text);
}
Give that a try