Either

if ( class_exists(\'MultiPostThumbnails\') ) :
    MultiPostThumbnails::the_post_thumbnail(get_post_type());
else:
    //someimage
endif;

OR

if ( class_exists(\'MultiPostThumbnails\') ) {
    MultiPostThumbnails::the_post_thumbnail(get_post_type());
} else {
    //someimage
}

In regards to your comment:

if ( class_exists(\'MultiPostThumbnails\') ) :

    $image = MultiPostThumbnails::get_the_post_thumbnail( get_post_type(), \'secondary-image\', NULL, \'proimg\', array(\'class\' => \'img-responsive imgwid\') );

    if ( !empty( $image ) ) {
        echo $image;
    } else {
        echo \'<p>asdasdasdasdasdas</p>\';
    }
endif;