Flutter validator syntax eror

Program is saying : The argument type String Function(String) can't be assigned to the parameter type String? Function(String?)?`. İn the validator are and under line , I Wrote error.

import 'package:asdasd/Validation/Student_Validator.dart';
import 'package:asdasd/models/Student.dart';
import 'package:flutter/material.dart';

class StudentAdd extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _StudentAddState();
  }
}

class _StudentAddState extends State with StudentValidationMixin {
  var student = Student.withoutinfo();
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Yeni öğrenci ekleme "),
      ),
      body: Container(
         margin: EdgeInsets.all(20.0),
        child: Form(
          child: Column(
            children: <Widget>[
              TextFormField(
                decoration: InputDecoration(
                    labelText: "Öğrencinin Adı", hintText: "Musa Develi"),
                    validator: validateFirstName,//eror
                    onSaved: (String value){//eror
                      student.firstName=value;//eror
                    },
              ),
            ],
          ),
        ),
      ),
    );
  }
}

class StudentValidation {
}