Flutter build_runner watch Not Working | The getter 'definingUnit' was called on null

To me, arpit solution worked: "Downgrading build_runner to 1.10.2".

(Added on 27 November 2020):

In other cases, the problem may arise if there is some import with "//" in it, eg.:

`import 'package:path//name.dart'.

Here is a regular expression to find the "import" that have "//" in the same line:

(import).*?(//).*?

This solution is taken from https://github.com/google/json_serializable.dart/issues/738, see dotdotcommadot answer.