When editing a node on a localized version I faced the next warnings (php8.1):
Warning: Trying to access array offset on value of type null in menu_form_node_form_alter() (line 691 of docroot/modules/menu/menu.module).
Same warnings for 702, 714, 728, 735, 746, 778 lines.
All these warnings (through they appear in the core menu module) are caused by setting $node->menu as NULL in the entity_translation_i18n_menu_node_menu_item_translate() in 295-298 lines
// Replace the menu item with the translated version, or null if there is
// no translated item. Store the original one in $node->source_menu.
$node->source_menu = $node->menu;
$node->menu = $menu;On PHP7.4 in was a notice.
I suggest to save the $node->menu array structure:
$node->source_menu = $node->menu;
foreach ($node->menu as $key => $item) {
$node->menu[$key] = $menu[$key] ?? NULL;
}