Addon-backgrounds: Fix grid disable

[shilman]

Issue: #13173

What I did

Fix backgrounds grid disable

How to test

Couldn't repro, but this will probably fix it. Self-merging @yannbf

[deep-cognite]

Thanks for this! Spent ages yesterday upgrading from v5 to v6 where I kept getting this error! From what I could tell, it happened if 'preview.js' didn't export 'parameters'.

Taking this message to pinpoint another thing that was bit "hard" to figure out during the transition from a custom webpack file to using main.js: and that was to add the custom plugins, as such:
webpackFinal: (config) => { return { ...config, plugins: [...config.plugins, ...custom.plugins], module: { ...config.module, rules: custom.module.rules } }; }

The example doesn't include 'plugins'

Also taking this comment to ask why under "About your Storybook" it says: "Storybook 6.1.0 is available. Upgrade from 6.1.0-rc.6 now." when I currently have 6.1.1 running 🤔