Android Studio – Flutter Throw or Return and Class Construction

Hello i started to learn Flutter with AndroidStudio IDE.
While i following a video to learn i change my code like in video.
But there is diffrences and this diffrences getting me errors.

import 'package:flutterogrencitakip/models/student.dart';
import 'package:flutter/material.dart';

final Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        theme: ThemeData.dark().copyWith(scaffoldBackgroundColor: darkBlue),
        debugShowCheckedModeBanner: false,
        home: HomeScreen());
  }
}

class HomeScreen extends StatelessWidget {
  List<Student> students = [
    Student.withId(1, "Yusuf", "Erarslan", 95),
    Student.withId(2, "Yusufff", "Erarslassn", 35),
    Student.withId(3, "YusufffAAA", "ErAAAarslassn", 15)
  ];
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        appBar: AppBar(
          title: Text("Öğrenci Takip Sistemi"),
        ),
        body: buildBody());
  }

  Widget buildBody() {
   return Column(
     children:<Widget>[
       Expanded(
         child: ListView.builder(
           itemCount: students.length,
             itemBuilder: (BuildContext context, int index){
             return Text(students[index].firstName);
             }),
       )
     ],
   );

  }
}

This code getting me error even i have student.dart in models folder.
This is output.

lib/main.dart:40:42: Error: The argument type 'String?' can't be assigned to the parameter type 'String' because 'String?' is nullable and 'String' isn't.
             return Text(students[index].firstName);

is this because of im using return instead of throw or my void main class and class myApp classes are has wrong definition ?
Because it was like that.

void main(){
const MyApp bla bla bla i can't remember exactly
}

Screenshots

My Project Folders:

My Problems:

Changed area by me:

My Student Class:

class Student{
  int? id;
  String? firstName;
  String? lastName;
  int? grade;
  String? status;

  (String firstName, String lastName, int grade){
    this.firstName= firstName;
    this.lastName= lastName;
    this.grade  = grade;
  }

  //named constructor
  Student.withId(int id,String firstName, String lastName, int grade){
    this.id=id;
    this.firstName= firstName;
    this.lastName= lastName;
    this.grade  = grade;
  }
}