Error: The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'

class StudentValidationMixin {

    String validateFirstName(String value) {
      if(value.length<2){
        return 'İsim en az iki karakter olmalıdır';
    }
  }
}

The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.

class _StudentAddState extends State with StudentValidationMixin {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("Yeni öğrenci ekle"),
      ),
      body: Container(
        margin: EdgeInsets.all(20.0),
        child: Form(
          child: Column(
            children: <Widget>[
              TextFormField(
                decoration: InputDecoration(labelText: "Öğrenci Adı", hintText: "Fatih"),
                validator: validateFirstName,
              ),
            ],
          ),
        ),
      ),
    );
  }
}

The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'.

These two separate bits of code are getting an error. What should I do?

The body might complete normally, causing 'null' to be returned, but the return type is a potentially non-nullable type.

When you don't return anything inside a function/method, it implicitely returns null, so this

String validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}

is equivalent to this

String validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
  return null;
}

But the return type of this method is String, a non-nullable type. To make it nullable (since you're returning null), add a question mark after it:

//    v
String? validateFirstName(String value) {
  if(value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}

The argument type 'String Function(String)' can't be assigned to the parameter type 'String? Function(String?)?'.

The validator parameter of TextFormField is a function that receives a nullable type and returns a nullable type (i.e. a String? Function(String?), so just add a question mark after the value parameter:

//                              v
String? validateFirstName(String? value) {
  if (value == null) {
    // The user haven't typed anything
    return "Type something, man!";
  }
  if (value.length<2){
    return 'İsim en az iki karakter olmalıdır';
  }
}