Opaque error for invalid CSS after upgrade: TypeError: Cannot read property '0' of undefined

[buildbreakdo]

After upgrading from react-scripts 1.1.4 to 2.1.3 see an opaque error message when trying to build:

Error in   TypeError: Cannot read property '0' of undefined

Thought this might be related to #2069 (determined to be bug of package managers?), but after trial and error determined it was related to invalid CSS.

Reproduce (react-scripts 2.1.3):

1. Add invalid CSS to any .css file, like:

label[for=*] {
  margin-right: 12px;
}

2. yarn build

Output:

Error in   TypeError: Cannot read property '0' of undefined

This invalid CSS does not produce a build error in react-scripts 1.1.4, so this is something people will bump into when they upgrade and not really know what to do (e.g., claim the upgraded version is broken).

[mrmckeb]

Interesting, that is a mysterious error message @buildbreakdo. Thanks for tracking down the cause.

Was it telling you that a JS or CSS file had an error? Can you supply an example in a repo? Thanks!

[buildbreakdo]

@mrmckeb Yes, indeed it is! This is what the full output looks like:

Here is an example repo:
https://github.com/buildbreakdo/css-breaks-create-react-app-demo

Clone and yarn build to see the error message. Line that causes this is here (invalid CSS):
https://github.com/buildbreakdo/css-breaks-create-react-app-demo/blob/master/src/App.css

[mrmckeb]

OK, so it looks like this need to be properly investigated ad solved. Would you be able to raise a PR? If not, we can leave it open for someone to pick up (perhaps me on the weekend).

Thanks for the repo too, that's great help.

[buildbreakdo]

@mrmckeb Yes, I'll give it a go, see if I can get a PR in sometime this week.

[buildbreakdo]

@mrmckeb Pull request with a fix is in