class FilterNull

class FilterNull extends FilterBase {
    
    /**
     * Tracks if an alert about this filter has been logged.
     *
     * @var bool
     */
    protected $logged = FALSE;
    
    /**
     * {@inheritdoc}
     */
    public function __construct(array $configuration, $plugin_id, $plugin_definition) {
        // Once per filter, log that a filter plugin was missing.
        if (!$this->logged) {
            $this->logged = TRUE;
            \Drupal::logger('filter')->alert('Missing filter plugin: %filter.', [
                '%filter' => $plugin_id,
            ]);
        }
        parent::__construct($configuration, $plugin_id, $plugin_definition);
    }
    
    /**
     * {@inheritdoc}
     */
    public function process($text, $langcode) {
        return new FilterProcessResult('');
    }
    
    /**
     * {@inheritdoc}
     */
    public function getHTMLRestrictions() {
        // Nothing is allowed.
        return [
            'allowed' => [],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function tips($long = FALSE) {
        return $this->t('Missing filter. All text is removed');
    }

}