Add title to breadcrumbs in Drupal 8

custom.theme.php

<?php

/**
 * Implements hook_process_HOOK().
 */
function custom_preprocess_breadcrumb(&$variables) {
  /** @var \Drupal\Core\Controller\TitleResolverInterface $title_resolver */
  $title_resolver = \Drupal::service('title_resolver');
  /** @var \Drupal\Core\Routing\CurrentRouteMatch $current_route */
  $current_route = \Drupal::service('current_route_match');
  $title = $title_resolver->getTitle(\Drupal::request(), $current_route->getRouteObject());
  $variables['breadcrumb'][] = ['text' => (string) $title];
}