code: public function getFlash($key = null)
      {
          if (! is_null($key)) {
              $value = null;
  
              if ($this->hasFlash($key)) {
                  $value = $this->get('_nur_flash')[$key];
                  unset($_SESSION['_nur_flash'][$key]);
              }
  
              return $value;
          }
  
          return $key;
      }
docs: Getting Flash Message
  
  @param string $key
  
  @return null|string