I was getting the same fatal error after creating a similar dynamic menu. What fixed the issue for me was redefining the default values:
$post->target="";
$post->attr_title="";
$post->description = '';
$post->classes="";
$post->xfn = '';
$post->status="publish";
$post->original_title="";
So your foreach loop would look like this:
foreach ( get_posts( $category_ten_last_posts ) as $post ) {
$post->menu_item_parent = $item->ID;
$post->post_type="nav_menu_item";
$post->object="header-lasts";
$post->type="header-lasts";
$post->menu_order = ++$menu_order;
$post->title = get_the_post_thumbnail( $post->ID ) . ' ' .$post->post_title;
$post->url = get_permalink( $post->ID );
$post->target="";
$post->attr_title="";
$post->description = '';
$post->classes="";
$post->xfn = '';
$post->status="publish";
$post->original_title="";
$child_items[]= $post;
}