DrupalPractice warns about using $this in trait

Checking my code with DrupalPractice standard I found code sniffer warns for every line contains $this or any other properties defined by the same trait.

Here my example trait:

/**
 * @file
 * Contains \Drupal\MY_MODULE\LangcodeTrait.
 */

namespace Drupal\MY_MODULE;

/**
 * A trait containing helper methods for language handling.
 */
trait LangcodeTrait {

  /**
   * Name of language.
   *
   * @var string
   */
  protected $langcode;

  /**
   * A fully-populated language object.
   *
   * @var \Drupal\core\Language\LanguageInterface|null
   */
  protected $lang;

  /**
   * Select language.
   *
   * @param string $langcode
   *   Language code.
   *
   * @return $this
   *   Current instance.
   */
  public function setLangcode($langcode) {
    $this->langcode = $langcode;
    $this->lang = \Drupal::languageManager()->getLanguage($this->langcode);
    return $this;
  }

  /**
   * Get code of currently active language.
   *
   * @return string
   *   Language code.
   */
  public function getLangcode() {
    if (!isset($this->langcode)) {
      $lang = \Drupal::languageManager()->getCurrentLanguage();
      $this->setLangcode($lang->getId());
    }

    return $this->langcode;
  }

}

Result:

FILE: [...]/src/LangcodeTrait.php
----------------------------------------------------------------------
FOUND 0 ERRORS AND 9 WARNINGS AFFECTING 8 LINES
----------------------------------------------------------------------
 19 | WARNING | Variable $langcode is undefined.
 26 | WARNING | Variable $lang is undefined.
 38 | WARNING | Variable $this is undefined.
 39 | WARNING | Variable $this is undefined.
 39 | WARNING | Variable $this is undefined.
 40 | WARNING | Variable $this is undefined.
 50 | WARNING | Variable $this is undefined.
 52 | WARNING | Variable $this is undefined.
 55 | WARNING | Variable $this is undefined.