How to return and persist the null in a factory constructor?

I have a problem with the migration to null-safety of flutter, I know that in a factory it should no longer return null so I return throw ArgumentError(""), however in my services I bring the value of canvas as null and that is where it sends the error, but what I want is to continue the application and leave that value as null.

To explain it better; I have two models, ModelA has a variable called canva of type ModelB. When I send a data mapping to ModelA, from the server the variable canva arrives null, so it enters a condition and gives an error.

ModelA :

class ModelA extends A {
   ModelA ({
       required ModelB? background,
       required ModelB? canvas,
     }) : super(
            background,
            canvas,
   );

    factory ModelA.fromMap(Map<String, dynamic>? map) {
        if (map == null) throw ArgumentError('ModelA map must not be null');
    
        return ModelA(
           background: ModelB.fromMap(map['background']), // go in with not-null service value
           canvas: ModelB.fromMap(map['canvas']), // go in with null service value
        );
      }
}

ModelB :

class ModelB extends B {
    ModelB({
        String? content,
      }) : super(
              content;
            );


    factory ModelB.fromMap(Map<String, dynamic>? map) {
        // * before the migration null-safety
        // if (map == null) return null
        if (map == null) throw ArgumentError('ModelB map must not be null'); // enter here (error)
    
        return ModelB(
          content: map['content'],
        );
      }
}

After the error, I enter my application without any problem, but it doesn't run any more my function...

This is where I do the programming to save and return my complete ModelA.

final someList = List<ModelA>.from(
        listFromServer.where((i) {
          return (i['someType'] == 1));
        }).map(
          (i) {
            print("enter here");
            final isBackOrCan = i['background'] ?? i['canvas'];
            if (isBackOrCan != null) {
              newListModelB
                  .add(ModelB.fromMap(isBackOrCan));
            }
            return ModelA.fromMap(i); // enter here the map background not null and also the canva null
          },
        ),
      );

[ ... more code that i want to continue ...]

} catch (e) {
      print(e); // the message
      throw CacheException();
}

Error :

If you want to preserve the old behavior of returning null, just use a static method instead of a factory constructor. (factory constructors provide almost no advantages to static methods anyway.) That would be the simplest and most straightforward fix.

But if you really want to disallow null, then ModelA.fromMap and ModelB.fromMap should require map to be non-nullable in the first place, and then you would need to proceed up the call chain to make callers check for null values. In your case:

class ModelA extends A {
  ModelA({
    required ModelB? background,
    required ModelB? canvas,
  }) : super(
          background,
          canvas,
        );

  factory ModelA.fromMap(Map<String, dynamic> map) {
    var background = map['background'];
    var canvas = map['canvas'];
    return ModelA(
      background: background == null
          ? null
          : ModelB.fromMap(background), // go in with not-null service value
      canvas: canvas == null
          ? null
          : ModelB.fromMap(canvas), // go in with null service value
    );
  }
}