I found a solution for a similar issue in my custom theme with HOOK_preprocess_links__language_block(&$variables) adapting the solution provided by the patch 3207515-fixed-indirect-modification-of-overloaded-element-03.patch in this issue https://www.drupal.org/project/field_group/issues/3207515 → .
Enabled 8.x-2.7 version on a fresh Drupal 10.1.5 (no more contributed or custom modules enabled). When editing a content type (to enable Gutenberg) I get the error
Warning: Trying to access array offset on value of type null in gutenberg_form_alter() (line 281 of modules/contrib/gutenberg/gutenberg.module).
but I manage to set it up. When I try to add a node the content area of the new node remains white... and that's all.
Afer installing 8.x-2.x-dev the error disapeared and adding a new node works like a charm.