Bad: Error: Field _prenom should be initialized because its type String doesnt allow null

I am creating an application with flutter and when I want to declare a variable and initialize it after, I have an error

Error: Field '_prenom' should be initialized because its type 'String' doesn't allow null.

import 'package:flutter/material.dart';

class LoginController extends StatefulWidget {
LoginControllerState createState() => new LoginControllerState();
}

class LoginControllerState extends State<LoginController> {

bool _log = true;
String _mail;
String _password;
String _prenom;
String _nom;

@override
Widget build(BuildContext context) {
// TODO: implement build
return new Scaffold(
  appBar: new AppBar(title: new Text("Authentification"),),
  body: new SingleChildScrollView(
    child: new Column(
      children: <Widget>[
        new Container(
          margin: EdgeInsets.all(20.0),
          width: MediaQuery.of(context).size.width - 40,
          height: MediaQuery.of(context).size.height / 2,
          child: new Card(
            elevation: 8.5,
            child: new Column(
              children: cardElements(),
            ),
          ),
        )
      ],
    ),
  ),
);
}

List<Widget> cardElements(){
List<Widget> widgets = [];

widgets.add(
  new TextField(
    decoration: new InputDecoration(hintText: "Entrez votre adresse mail"),
    onChanged: (string) {
      setState(() {
        _mail = string;
      });
    },
  )
);
widgets.add(
    new TextField(
      decoration: new InputDecoration(hintText: "Entrez votre mot de passe"),
      onChanged: (string) {
        setState(() {
          _password = string;
        });
      },
    )
);

if(_log == false ) {
  widgets.add(
      new TextField(
        decoration: new InputDecoration(hintText: "Entrez votre prenom"),
        onChanged: (string) {
          setState(() {
            _prenom = string;
          });
        },
      )
  );
  widgets.add(
      new TextField(
        decoration: new InputDecoration(hintText: "Entrez votre nom"),
        onChanged: (string) {
          setState(() {
            _nom = string;
          });
        },
      )
  );
}

widgets.add(
  new TextButton(
      onPressed: () {
        setState(() {
          _log = !_log;
        });
      },
      child: new Text(
          (_log == true)
              ? "Pour créer un compte, appuyez ici"
              : "Vous avez déjà un compte ? Appuyez ici"
      )
  )
);
return widgets;
}

}

Solution 1: Stuck

Dart is null safe . You either must always assign a value or mark it explicitly as nullable using a ?

String _prenom = "Something";

or

String? _prenom;

Solution 2: Zakaria Ferzazi

in Dart class always add ? after DataType

bool? log = true;
String? mail;
String? password;
String? prenom;
String? nom;```