Redirect DRAFT Posts to a Custom URL (Custom Field) : Currently 404 Page

Question

I am trying to create a function that REDIRECTs Draft and Trashed Posts to a custom field I have in all of my posts.

I am calling this function at the start of header.php, however the webpage just shows the standard 404 Page. With this I cannot call any custom fields I have. I can make it work if I have the POSTID but cannot get that on a 404 page.

Advice appreciated, I have spent several hours on this.

The following function was taken from another user I found.

//redirect any draft/trashed posts to custom postmeta field: redirect_url_on_draft
add_action('wp', 'custom_redirect_on_draft');
function custom_redirect_on_draft(){
    if ( !current_user_can( 'edit_pages' ) ) {
        if (is_404()){
            global $wp_query, $wpdb;
            $page_id = $wpdb->get_var( $wp_query->request );
            $post_status = get_post_status( $page_id );
                $post_redirect = get_post_meta( $page_id, 'redirect_url_on_draft', true );
            if(($post_status == 'trash' || $post_status == 'draft') && $post_redirect){
                wp_redirect($post_redirect, 302);
                die();
            }
        }
    }
}