Flutter : unhandled exception: invalid argument - api

Question

I'm trying to send two list of images file and i have an error with that and i don't know why this happen
this is the function which i use for that :
Future<dynamic> upload(List<File> imageFile, List<File> backID) async {
var headers =
Provider.of<LoginUserProvider>(context, listen: false).httpHeader;
var uri = Uri.parse(
"xyz");
var request = new http.MultipartRequest("POST", uri);
var requestCopy = new http.MultipartRequest("POST", uri);
request.headers.addAll(headers);
//request.headers.addAll(Environment.requestHeaderMedia);
for (int i = 0; i < uploadedFilesList.length; i++) {
var length = await imageFile[i].length();
var stream =
// ignore: deprecated_member_use
new http.ByteStream(DelegatingStream.typed(imageFile[i].openRead()));
var multipartFile = http.MultipartFile(
'back_id_photos[$i]',
stream,
length,
contentType: MediaType('application', 'x-tar'),
);
var multipartFilePhotos = new http.MultipartFile(
'photos[$i]',
stream,
length,
contentType: MediaType('application', 'x-tar'),
);
request.fields['section_id'] = VillaID.toString();
request.fields['start_date'] = requeststartDate;
request.fields['end_date'] = requestendDate;
request.fields['names'] = _nameController.toString();
request.fields['phones'] = _phoneController.toString();
request.fields['photos'] = multipartFilePhotos.toString();
request.fields['back_id_photos'] = multipartFile.toString();
//contentType: new MediaType('image', 'png'));
request.files.add(multipartFile);
request.files.add(multipartFilePhotos);
var response = await request.send();
print(response.statusCode);
response.stream.transform(utf8.decoder).listen((value) {
print(value);
});
var streamedResponse = await requestCopy.send();
var responses = await http.Response.fromStream(streamedResponse);
// ignore: unused_local_variable
final responseData = json.decode(responses.body) as Map<String, dynamic>;
print(json.decode(responses.body));
if (response.statusCode == 200 || response.statusCode == 201) {
return true;
}
}
}
and in the part of
var response = await request.send();
I have the error exactly before the dot (.) and this is the error which shown
unhandled exception: invalid argument(s) (input): must not be null
Can anyone tell me what should i do to solve that !

Answer 1

Send your Multipart Part Files as below and Use await http.Multipart as accessing file is async function and need a promise to run.
Try this:
await http.MultipartFile(
'back_id_photos[$i]',
stream,
length,
contentType: MediaType('application', 'x-tar'),
);
OR
request.files.add(new http.MultipartFile.fromBytes('file', await File.fromUri("<path/to/file>").readAsBytes(), contentType: new MediaType('image', 'jpeg')))
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