This change fixes an issue in the Widgets editor: widgets group was missing a `.wp-widget-group__inner-blocks` container.
Props noisysocks, ironprogrammer.
Merges [52730] to the 5.9 branch.
Fixes#55072.
git-svn-id: https://develop.svn.wordpress.org/branches/5.9@52731 602fd350-edb4-49c9-b593-d223f7449a82