this.reference is not working on flutter/dart

Question

I don't know what's the problem here but i get an error on [this.reference], it says "The parameter 'reference' can't have a value of 'null' because of its type, but the implicit default value is 'null'." When I add required it says "Can't have modifier 'required' here"

Here's the code for it:

import 'package:cloud_firestore/cloud_firestore.dart';

class Request {
  final String type;
  final String reason;
  DocumentReference reference;

  Request(this.reason, this.type, [this.reference]);

  String get requestId {
    return reference.id;
  }

  Map<String, dynamic> toMap() {
    return {"type": type, "reason": reason};
  }
}

Answer 1

Did u tried making those Nullable

final String? type;
final String? reason;

Also Try Giving default initial values

Request(String reason = '',String type = '');

Answer 2

Square brackets around a parameter make that parameter an optional positional parameter. Therefore when you write:

Request(this.reason, this.type, [this.reference]);

You are specifying that the Request constructor take two required positional parameters (reason and type) and an optional positional parameter (reference).

If you want reference be optional, it must have a default value if the caller doesn't provide it. Your options are:

• Provide a const, non-null default value:

  Request(
     this.reason,
     this.type,
     [this.reference = DocumentReference.someConstConstructor()],
  );
  

  (The DocumentReference class from package:cloud_firestore does not provide a const constructor, so you can't do this in your case, but I've listed it here for completeness.)
• Make reference nullable so that it can use null as an implicit default value:

  class Request {
    ...
    DocumentReference? reference;
    ...
    Request(this.reason, this.type, [this.reference]);
  

  or if you want a nullable parameter with a non-nullable member:

  class Request {
    ...
    DocumentReference reference;
    ...
    Request(this.reason, this.type, [DocumentReference? reference])
      : reference = reference ?? DocumentReference();
  

  The latter is a general technique for making parameters optional when the parameter type has no const constructors available.
• Make reference non-optional. Since using square brackets around a parameter make that parameter optional, adding the required keyword would be a contradiction and would make no sense. If you want it to be a required positional parameter, simply remove the square brackets:

  Request(this.reason, this.type, this.reference);
  

  Or if you want to be a required named parameter, use curly braces with the required keyword:

  Required(this.reason, this.type, {required this.reference});
  

I suggest reading the Functions Parameters section of the Dart Language Tour.