After adding a block file to src/Plugin/Block I can’t add the block on the Block Layout admin page. When I filter by block name I can see it, but then when I try to place it I can’t get to the configure block screen (pressing the ‘Place block’ button produces no effect). I do not get any errors.

What could be the reason for such a behaviour?

The block file contents:

    <?php  namespace DrupalCoreBlockBlockBase; use DrupalCoreLink; use DrupalCorePluginContainerFactoryPluginInterface; use DrupalCoreSessionAccountInterface; use DrupalcoreUrl; use SymfonyComponentDependencyInjectionContainerInterface;  /**  * Provides a block with a link to start the toru.  *   * @Block(  *    id = "iai_tour_link",  *    admin_label = @Translation("Link for the IAI Product Guide tour"),  *    category = @Translation("Links"),  *    context = {  *       "node" = @ContextDefinition(  *         "entity:node",  *          label = @Translation("Current Node")  *        )  *     }  *   )  */ class TourLinkBlock extends BlockBase implements ContainerFactoryPluginInterface{     /**      * The current user.      *       * @var DrupalCoreSessionAccountInterface;      *       */     protected $currentUser;      /**      * Constructs an TourLinkBlock object.      *       * @param array $configuration      * A configuration array containing information about the plugin instance.      * @param string $plugin_id      * The plugin_id for the plugin instance.      * @param mixed $plugin_definition      * The plugin implementation definition.      */     public function __construct(array $configuration, $plugin_id, $plugin_definition, AccountInterface $current_user){         parent::__construct($configuration, $plugin_id, $plugin_definition);         $this->currentUser = $current_user;     }      /**      * {@inheritdoc}      */     public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition){         return new static(             $configuration,             $plugin_id,             $plugin_definition,             $container->get('current_user')         );     }      public function build(){          $node = $this->getContextValue('node');         if ($this->currentUser->hasPermission('access tour') && $node->getType() == 'book'){             $url = Url::fromRoute('<current>', array(), array('query' => array('tour' => 1)));             $build['tour_link']= [                 '#type' => 'markup',                 '#markup' => Link::fromTextAndUrl(t('Take the tour!'), $url)->toString(),             ];             $build['#attached']['library'][] = 'tour/tour';             return $build;         }     } }