Invalid argument(s): join(null, "bin", "cache", "dart-sdk"): part 0 was null, but part 1 was not. code example

Example: Invalid argument(s): join(null, "bin", "cache", "dart-sdk"): part 0 was null, but part 1 was not.

#on mac
1) run:
$ pub global activate dartdoc

2) cd to root of project then run:
flutter pub global run dartdoc:dartdoc