Issues reported by phpStan

Line src/Plugin/Notifier/Manager.php
------ ----------------------------------------------------------------------
13 Plugin definitions cannot be altered.

This means the constructor needs to call alterInfo, and setCacheBackend. Like the WidgetPluginManager does below.

  /**
   * {@inheritdoc}
   */
  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
    parent::__construct('Plugin/facets/widget', $namespaces, $module_handler, WidgetPluginInterface::class, FacetsWidget::class);
    $this
      ->alterInfo('widget_plugin_info');
    $this
      ->setCacheBackend($cache_backend, 'facet_widget_plugins');
  }