Flutter validator syntax eror-CodePudding

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 {
}

CodePudding user response：

i'm guessing validateFirstName is a function, right? You have to make your validator function the same as the validator,

String? Function(String?)

String is not the same as String?, the first means the value cannot be null, the second means it can be null.

this happened because null-safety is enabled