url_launcher: 6.0.0-nullsafety.1 is missing web support (it just does not work on web, while the previous version does support web). This is intended, see below.

See its pubspec:

  # TODO(mvanbeusekom): Temporary disabled until web is migrated to nnbd (advised by @blasten).
  #url_launcher_web: ^0.1.3

I just figured it would be good to document this because it was non-trivial to figure this out.