I am also seeing this on edit.php in the Admin screen, It only happens when a Filter is selected.
The problem seems to be in inc/extras line 61
You can fix it by checking for an empty search
function decode_highlight_search_results( $text ) {
if ( is_search() ) {
$sr = get_search_query();
$keys = implode( '|', explode( ' ', get_search_query() ) );
if ($keys != '') {
$text = preg_replace( '/(' . $keys .')/iu', '<mark class="search-highlight"></mark>', $text );
}
}
return $text;
}