Package not returning data? - flutter

Question

im currently having an issue using a package, the basic function it does it uploads and image and bring backs the url of the image, im not having issues uploading images (i printed the info) but whenever i do the return to bring back the information to my actual app it just brings back null.
The first code is where i call the function to upload the image and bring it back and also it eventually ataches it to another function, this part is found in the actual app.
File file, OnCreatePetEvent event, Emitter<PetState> emit) async {
PetsRepository repository = PetsRepository();
try {
ImageFileModel fileModel = await repository.putImage(file);
print(fileModel.toJson());
fileModel.toJson();
event.petModel.image = fileModel.data.imageKey;
event.petModel.thumbnail = fileModel.data.thumbnailKey;
repository.createPet(event.petModel);
} catch (e) {
throw e;
}
}
The second code is the function where i upload the image and eventually returns it, as previously said it does upload it and sends back data, this part is found in the package.
final apiProvider = Api.instance;
await apiProvider.postFile('upload_image', file, true).then((value) async {
dynamic json = jsonDecode(await value.stream.bytesToString());
print(json);
print(json['message']);
if (value.statusCode == 200) {
ImageFileModel filesURLs = ImageFileModel.fromJson(json);
print(filesURLs.toJson());
return filesURLs;
} else {
return json['message'];
}
}).catchError((e) {
print('error uploading file: ${e}');
});
}
ImageModel
//
// final imageFileUiModel = imageFileUiModelFromJson(jsonString);
import 'dart:convert';
ImageFileModel imageFileModelFromJson(String str) =>
ImageFileModel.fromJson(json.decode(str));
String imageFileModelToJson(ImageFileModel data) => json.encode(data.toJson());
class ImageFileModel {
ImageFileModel({
required this.message,
required this.data,
});
String message;
Data data;
factory ImageFileModel.fromJson(Map<String, dynamic> json) => ImageFileModel(
message: json["message"],
data: Data.fromJson(json["data"]),
);
Map<String, dynamic> toJson() => {
"message": message == null ? null : message,
"data": data == null ? null : data.toJson(),
};
}
class Data {
Data({
this.imageKey,
this.thumbnailKey,
});
String? imageKey;
String? thumbnailKey;
factory Data.fromJson(Map<String, dynamic> json) => Data(
imageKey: json["image_key"] == null ? null : json["image_key"],
thumbnailKey:
json["thumbnail_key"] == null ? null : json["thumbnail_key"],
);
Map<String, dynamic> toJson() => {
"image_key": imageKey == null ? null : imageKey,
"thumbnail_key": thumbnailKey == null ? null : thumbnailKey,
};
}
Console log
I/flutter (18011): success
I/flutter (18011): {message: success, data: {image_key: images/571221DTO.jpeg, thumbnail_key: images/53221fFX.jpeg}}
E/flutter (18011): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: type 'Null' is not a subtype of type 'ImageFileModel'```