How to get the url from Firebase Storage file?
You have to await for the URL when calling ref.getDownloadURL()
change
return ref.getDownloadURL();
into
return await ref.getDownloadURL();
The UploadTask class extends Future, so you can also use await there already:
await ref.putData(signature!);
final downurl = await ref.getDownloadURL();
final url = downurl.toString();