'appendChild' of undefined when adding css

[Gujionsen]

Hi, After adding content-script.css I'm getting this error on development build

vue.config:

contentScripts: {
entries: {
'content-script': [
'src/content-scripts/content-script.css',
'src/content-scripts/content-script.js',
]
}
}

Interesting is that when css record is added in manifest and it is built for production, everything is fine.
Some help will be very appreciated!