[BUG] Search suggestions don't work

[stweil]

Description

Search suggestions don't work.

Reproduction

Steps to reproduce the behaviour:

1. Add the search plugin and enable search suggestions.
2. Enter text in the search field (3 or more characters).
3. See error: there are no search suggestions

Expected Behavior

Search suggestions should be shown.

Environment

• OS version: Debian GNU/Linux trixie
• Apache Solr version: 9
• TYPO3 version: 12

[stweil]

Related error in the TYPO3 log (occurs as soon as something is written in the search field):

 Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1476107295: PHP Warning: Undefined array key "solrcore" in /viewer/vendor/kitodo/presentation/Classes/Middleware/SearchSuggest.php line 56 | TYPO3\CMS\Core\Error\Exception thrown in file /viewer/vendor/typo3/cms-core/Classes/Error/ErrorHandler.php in line 141. Requested URL: https://DOMAIN/

[stweil]

The issue still occurs with 6.0.0-rc1. As soon as the third character is typed in the search field, a TYPO3 execption is thrown:

 Core: Exception handler (WEB): Uncaught TYPO3 Exception: #1476107295: PHP Warning: Undefined array key "solrcore" in /viewer/vendor/kitodo/presentation/Classes/Middleware/SearchSuggest.php line 56 | TYPO3\CMS\Core\Error\Exception thrown in file /viewer/vendor/typo3/cms-core/Classes/Error/ErrorHandler.php in line 141. Requested URL: https://DOMAIN/