Storybook: @storybook/[email protected] doesn't work with import inside css in react-scripts@^1

As I wrote in #1073 (I've being asked to open a new issue):

Might related to #1027

But, it doesn't compatible with react-scripts@^1 when it comes to CSS. If you have a CSS files that import other CSS files, it breaks with this error:

ERROR in ./~/css-loader?{"importLoaders":1}!./~/postcss-loader/lib?{"plugins":[null]}!./src/stories/blah.css
Module build failed: TypeError: Cannot read property 'postcss' of null

Code example:

/* a.css */
@import "./b.css";

/* b.css */

:root {
  /* ... */
}

// Story file
import "./a.css";

If there is more information needed, please tell me.