There has been a new update in dart language named null safety. So with this update, many of the recent packages and libraries which were made before this update, are not still integrated and compatible with this update and they are outdated. you should Wait for the packages that you depend on to migrate. read here for more information:
https://dart.dev/null-safety/migration-guide
If you want to run your application without null safety you can use this command flutter run --no-sound-null-safety. for more information about this visit here: https://dart.dev/null-safety/unsound-null-safety
solved The library ‘package:flutter_mobile_vision/flutter_mobile_vision.dart’ is legacy, and should not be imported into a null safe library [closed]