TYPO3 Core

TCA is as following (shorted)

        'fieldname' => [
            'exclude' => 0,
            'l10n_mode' => 'exclude',
            'l10n_display' => 'defaultAsReadonly',
            'label' => 'A label,
            'config' => [
                'type' => 'group',
                'internal_type' => 'db',
                ...
            ],
        ],

How to reproduce:

1) Create a record in the default language
2) Localize the record in list module

In order to quickly reproduce the problem, you can install ext:news and change to TCA for the field "related" of the table "tx_news_domain_model_news" as shown above.

Result:

The following PHP exception is thrown:

PHP Warning: Invalid argument supplied for foreach() in /path/to/typo3/sysext/backend/Classes/Form/Element/GroupElement.php line 158

The $selectedItems value is `0` instead of an array resulting in the error shown above