Saving to Firebase storage not possible in an isolate - firebase

Question

I have an app feature where the user picks images from his phone and then uploads them to Firebase Storage.
I thought that the upload process should be done in a separate isolate.
I keep getting an exception which I think is related to the Multi Image Picker package.
The exception is:
E/flutter (12961): [ERROR:flutter/lib/ui/ui_dart_state.cc(177)]
Unhandled Exception: Exception: NoSuchMethodError: The getter
'defaultBinaryMessenger' was called on null.
When the user presses on the upload button, this method is called:
Future<void> _initIsolate() async {
ReceivePort receivePort = ReceivePort();
receivePort.listen(
(message) {
print(message.toString());
},
onDone: () => print('Done'),
onError: (error) => print('$error'),
);
await compute(
_function, // This function is called in the separate isolate
{
'sendingPort': receivePort.sendPort,
'images': images,
},
);
}
The _function method is as follows:
static void _function(Map<String, dynamic> parameterMap) async {
SendPort sendingPort = parameterMap['sendingPort'];
List<Asset> images = parameterMap['images'];
List<String> urls = [];
int index = 0;
images.forEach(
(image) async {
String url = await getDownloadUrl(image); // a helper method
urls.add(url);
sendingPort.send('Image number: $index uploaded');
index += 1;
},
);
final CollectionReference collectionRef = FirebaseFirestore.instance.collection('offers');
final user = CurrentUser.getCurrentUser();
await collectionRef.doc(user.uid).set(
{
'time': FieldValue.serverTimestamp(),
'urls': urls,
},
);
}
The helper method _getDownloadUrl is as follows:
Future<String> getDownloadUrl(Asset image) async {
String rannum = Uuid().v1();
final ByteData byteData = await image.getByteData(); // --> This produces a defaultBinaryMessenger
final List<int> imageData = byteData.buffer.asUint8List();
Reference reference = FirebaseStorage.instance.ref().child("offers/$rannum");
UploadTask uploadTask = reference.putData(imageData);
TaskSnapshot downloadUrl = await uploadTask.whenComplete(() => null);
Future<String> futureUrl = downloadUrl.ref.getDownloadURL();
return futureUrl;
}
The getByteData method is part of the multi_image_picker package.
The source code is:
Future<ByteData> getByteData({int quality = 100}) async {
if (quality < 0 || quality > 100) {
throw new ArgumentError.value(
quality, 'quality should be in range 0-100');
}
Completer completer = new Completer<ByteData>();
ServicesBinding.instance.defaultBinaryMessenger // --> Exception here. ServicesBinding.instance is null
.setMessageHandler(_originalChannel, (ByteData message) async {
completer.complete(message);
ServicesBinding.instance.defaultBinaryMessenger
.setMessageHandler(_originalChannel, null);
return message;
});
await MultiImagePicker.requestOriginal(_identifier, quality);
return completer.future;
}
Why is the ServicesBinding.instance null?
Since this method is working fine without using Isolates, does this have something to do with the isolates?