Cannot retrieve length of file, path = 'null' (OS Error: No such file or directory, errno = 2)

I'm new migrating to flutter newest version. I've been migrating Flutter v1 to v3, but when I use the plugin file_picker: ^5.0.1 gallery and camera are working fine, but whenever is an file, this error appears, I've been searching everywhere and I haven't found a solution for this, does anyone know how I can fix this problem?

 var request = http.MultipartRequest('POST', Uri.parse(baseURL! + urlFiles));
request.fields['apiKey'] = API_KEY;
request.fields['username'] = username as String;
request.fields['authenticationCode'] = authCode as String;
request.files.add(await http.MultipartFile.fromPath('myfile', filename)); //<-- Error

var streamedResponse = await request.send();
final response = await http.Response.fromStream(streamedResponse);

// If the response from the server is correct code == 200 we proceed with the parsing of the json
if (response.statusCode == 200) {
  final responseJson = json.decode(response.body);

This is how I've been working files and images:

  _pickFile() async {

FilePickerResult? result = await FilePicker.platform.pickFiles();
if (result == null) return;

PlatformFile file = result.files.first;

print('File Name: ${file.name}');
print('File Size: ${file.size}');
print('File Extension: ${file.extension}');
print('File Path: ${file.path}');
//   if (path == null) return;
Navigator.pop(context);
_uploadFile('$path');}

_getPhoto(ImageSource source) async {
    var image = await ImagePicker().getImage(source: source, imageQuality: 50);

    if (image == null) return;

    path = image.path;
    Navigator.pop(context);
    if (kDebugMode) {
      print('---> ImagePicker $path');
    }
    _uploadFile(path as String);}

At the end I solved it by adding these new lines and everything is working as expected and it's received by the server.

  _pickFile() async {
final path = await FilePicker.platform.pickFiles(allowMultiple: true);
String? filePath = path?.files.single.path;

if (path == null) return;
Navigator.pop(context);
_uploadFile('$filePath');
}