A Drupal 8 block plugin which needs to be shown only on node pages (using context)

TextAdsBlock.php

<?php

namespace Drupal\my_module\Plugin\Block;

use Drupal\Core\Access\AccessResult;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Drupal\Core\Session\AccountInterface;
use Drupal\node\NodeInterface;
use Drupal\my_module_lists\ProfilesService;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a 'TextAdsBlock' block.
 *
 * @Block(
 *   id = "text_ads",
 *   admin_label = @Translation("Text Ads"),
 *   category = @Translation("Ads"),
 *   context_definitions = {
 *     "node" = @ContextDefinition("entity:node", label = @Translation("Node"))
 *   }
 * )
 */
class TextAdsBlock extends BlockBase implements ContainerFactoryPluginInterface {

  /**
   * Drupal\my_lists\ProfilesService definition.
   *
   * @var \Drupal\my_lists\ProfilesService
   */
  protected $profilesService;

  public function __construct(
    array $configuration,
    $plugin_id,
    $plugin_definition,
    ProfilesService $profiles_service
  ) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->profilesService = $profiles_service;
  }

  /**
   * {@inheritdoc}
   */
  public function blockAccess(AccountInterface $account) {
    return AccessResult::allowedIf($this->getContextValue('node')->bundle() == 'my_node_type');
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('my_lists.profiles')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function build() {
    $location = $this->getLocation($this->getContextValue('node'));
    $profiles = empty($location)
      ? $this->profilesService->getProfiles()
      : $this->profilesService->getProfilesInLocation($location['country_code'], $location['administrative_area'], $location['locality']);

    shuffle($profiles);

    // If there are no profiles, we don't want to render anything at all.
    return count($profiles) ? [
      '#theme' => 'text_ads',
      '#profiles' => array_slice($profiles, 0, 20),
    ] : NULL;
  }

  /**
   * {@inheritdoc}
   *
   * Let's cache this block for a definite time.
   */
  public function getCacheMaxAge() {
    return 43200;
  }

  protected function getLocation(NodeInterface $node) : array {
    $location = $node->get('field_location')->getValue();
    return $location ? $location[0] : [];
  }

}