When the MediaSearch extension is not installed, creating an inputbox with searchengine=MediaSearch results in a notice.
<inputbox> type=search searchengine=MediaSearch </inputbox>
Notice: Did not find alias for special page 'MediaSearch'. Perhaps no aliases are defined for it? [Called from MediaWiki\SpecialPage\SpecialPageFactory::getLocalNameFor in mediawiki/includes/SpecialPage/SpecialPageFactory.php at line 1876] in mediawiki/includes/Debug/MWDebug.php on line 486
Instead, it should fall back to using Special:Search.