Custom post type title is set by other custom post type

I found that i just have to use

$post->post_name 

instead of

$post->post_title 

because $post->post_name holds the slug also the autogenerated if you save as draft without defining a title.

So the code would as follows

global $post;
$slug = $post->post_name;
echo "<script>alert ('".$title.")</script>";
$args = array(
   'post_type' => 'traener',
   'posts_per_page' => -1,
   'orderby' => 'post_title', // meta_value',
   'order'   => 'ASC',
);
$my_query = new WP_Query($args);
if ($my_query->have_posts()) {
    while ($my_query->have_posts()) : $my_query->the_post();
        if ($selectedtrainer == get_the_title()) {
           echo '<option value="'.esc_html( get_the_title() ).'"selected=selected>'.esc_html( get_the_title() ).'</option>';                           
        } else {
            echo '<option value="'.esc_html( get_the_title() ).'">'.esc_html( get_the_title() ).'</option>'; 
        } 
    endwhile; 
}
wp_reset_postdata();

$post->post_name = $slug;

Hope this can be of some use for others.