title: Missing concrete of implementation of 'statefulwidget.createstate' and setstate isn't deifned
body: <p>I'm totally new in flutter just learned last weeks, and facing two problem here, I'm trying to solve the error, but still can't able to solve it, can anyone help on me? Below is the dart code.</p>
  <p>'main.dart'</p>
  <pre><code>class Sharer extends StatefulWidget
  {
    bool _isLoggedIn = false;
    Map userProfile;
    final facebookLogin = FacebookLogin();
    @override
    Widget build(BuildContext context)
    {
      return Scaffold(
        appBar: AppBar(
          backgroundColor: Colors.amber,
          centerTitle: true,
          title: Text('Welcome Sharer',
            style: TextStyle(
              fontSize: 16.0,
              color: Colors.black87,
              letterSpacing: 1.0,
            ),
          ),
        ),
        body:
        //Center(
          //child: isLoggedin
              //?
          Row(
            mainAxisAlignment: MainAxisAlignment.center,
            crossAxisAlignment: CrossAxisAlignment.center,
            children: &lt;Widget&gt;[
              Expanded(
                flex: 1,
                child: Container(
                  padding: EdgeInsets.all(20.0),
                  child: RaisedButton.icon(
                    onPressed: () async {
                      final result = await facebookLogin.logIn(['email']);
  
                      switch (result.status) {
                        case FacebookLoginStatus.loggedIn:
                          final token = result.accessToken.token;
                          final graphResponse = await http.get('https://graph.facebook.com/v2.12/me?fields=name,picture,email&amp;access_token=${token}');
                          final profile = JSON.jsonDecode(graphResponse.body);
                          print(profile);
                          setState(() {
                            userProfile = profile;
                            _isLoggedIn = true;
                          });
                          break;
  
                        case FacebookLoginStatus.cancelledByUser:
                          setState(() =&gt; _isLoggedIn = false );
                          break;
                        case FacebookLoginStatus.error:
                          setState(() =&gt; _isLoggedIn = false );
                          break;
                      }
                      
  
                    },
                    icon: Icon(
                      MyFlutterApp.facebook,
                    ),
                    label: Text(
                      'Sharer',
                      style: TextStyle(
                        fontFamily: 'MyFlutterApp',
                        color: Colors.black87,
                        letterSpacing: 1.0,
                      ),
                    ),
                  ),
                ),
              ),
            ],
          ),
        //),
      );
    }
  }
  </code></pre>
  <p>The error was shown at the class name say 'Missing concrete implementation of statefulwidget.create state' another is 'setstate isn't define', i do not know what are the problem can someone help out?
  thanks and regards.</p>