The method 'validate' was called on null. Receiver: null Tried calling: validate()

[abhisondagar605]

════════ Exception caught by widgets library ═══════════════════════════════════════════════════════
The following NoSuchMethodError was thrown building CreateAccountPage(dirty, dependencies: [_InheritedTheme, _LocalizationsScope-[GlobalKey#0560e]], state: _CreateAccountPageState#8ae4d):
The method 'validate' was called on null.
Receiver: null
Tried calling: validate()

The relevant error-causing widget was:
CreateAccountPage file:///C:/Users/Abhi%20Sondagar/AndroidStudioProjects/instagram_app/lib/pages/HomePage.dart:44:51
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1 _CreateAccountPageState.submitUserName (package:instagramapp/pages/CreateAccountPage.dart:18:14)
#2 _CreateAccountPageState.build (package:instagramapp/pages/CreateAccountPage.dart:88:26)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:4619:28)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4502:15)
...
════════════════════════════════════════════════════════════════════════════════════════════════════
D/libc-netbsd(27927): [getaddrinfo]: hostname=firestore.googleapis.com; servname=(null); app_uid=10624; ai_addrlen=0; ai_canonname=(null); ai_flags=1024; ai_family=0 from prox result 0

[abhisondagar605]

code of createaccountpage class

import 'dart:async';

import 'package:flutter/material.dart';
import 'package:instagramapp/widgets/HeaderWidget.dart';

class CreateAccountPage extends StatefulWidget {
@OverRide
_CreateAccountPageState createState() => _CreateAccountPageState();
}

class _CreateAccountPageState extends State {
final _scaffoldKey = GlobalKey();
final _formKey = GlobalKey();
String userName;

submitUserName() {
final form = _formKey.currentState;
if (form.validate()) {
form.save();

  SnackBar snackBar = SnackBar(content: Text('welcome ' + userName));
  _scaffoldKey.currentState.showSnackBar(snackBar);
  Timer(Duration(seconds: 4), () {
    Navigator.pop(context, userName);
  });
}

}

@OverRide
Widget build(BuildContext parentContext) {
print('account page');
return Scaffold(
key: _scaffoldKey,
appBar:
header(context, strTitle: 'Settings', disappearedBackButton: true),
body: ListView(
children: [
Container(
child: Column(
children: [
Padding(
padding: EdgeInsets.only(top: 26.0),
child: Text(
'set yp a username',
style: TextStyle(
fontSize: 26.0,
),
),
),
Padding(
padding: EdgeInsets.all(17.0),
child: Container(
child: Form(
key: _formKey,
autovalidate: true,
child: TextFormField(
style: TextStyle(
color: Colors.white,
),
validator: (val) {
if (val.trim().length < 5 || val.isEmpty) {
return 'user name is very short';
} else if (val.trim().length > 15) {
return 'user name is very long';
} else {
return null;
}
},
onSaved: (val) => userName = val,
decoration: InputDecoration(
enabledBorder: UnderlineInputBorder(
borderSide: BorderSide(color: Colors.grey),
),
focusedBorder: UnderlineInputBorder(
borderSide: BorderSide(color: Colors.white),
),
border: OutlineInputBorder(),
labelText: 'User name',
labelStyle: TextStyle(fontSize: 16.0),
hintText: 'must be atleast 5 character',
hintStyle: TextStyle(color: Colors.grey),
),
),
),
),
),
GestureDetector(
onTap: submitUserName(),
child: Container(
height: 55.0,
width: 360.0,
decoration: BoxDecoration(
color: Colors.green,
borderRadius: BorderRadius.circular(8.0),
),
child: Center(
child: Text(
'Proceed',
style: TextStyle(
color: Colors.white,
fontSize: 16.0,
fontWeight: FontWeight.bold,
),
),
),
),
)
],
),
),
],
),
);
}
}