get_post_types() is returning null

You have to return your formatted array. Change the code like below

function get_posttype_list() : array {
    $pt_list = [];
    $post_types = get_post_types( array( 'public' => true ), 'objects' );

    foreach( $post_types as $pt ) {
        $pt_list[ $pt->name ] = $pt->labels->singular_name;
    }
    return $pt_list;
}