Flutter: Unable to load assets for flutter_map

I'm building an app where I embed a map using flutter_map. I have downloaded offline tiles, but they cannot be loaded from the assets folder because the following error appears: Error. flutter_map itself gives the following errors: Error

My main.dart file: Main Dart File

My pubspec.yaml file: Pubspec.yaml

My folder structure:

I solved my problem by simply adding each folder individually. Apparently the developers of Flutter don't offer any other option at the moment.