functions.php

<?php
function preventUnnamedPosts($postId) {
  $post = get_post($postId);
  if(empty($post->post_title)) {
    remove_action( 'save_post', 'set_private_categories' );
    wp_update_post(array(
      'ID' => $postId,
      'post_title' => $postId
    ));
  }
}

add_action( 'save_post', 'preventUnnamedPosts');