Undefined variable $entity warning on non-special pages

This is an invalid issue that stems from changing to the 2.x branch and applying a patch from issue 2949963 which added this code:

elseif ($entity && ($entity instanceof ContentEntityInterface) && !$entity->isNew()) {
      $entity_url = base_path() . $entity->toUrl('canonical', ['absolute' => TRUE])->getInternalPath();
      $system_config = \Drupal::configFactory()->get('system.site');

      if ($system_config->get('page.front') === $entity_url) {
        $metatags = $this->metatagDefaults->load('front');
      }
      elseif ($system_config->get('page.403') === $entity_url) {
        $metatags = $this->metatagDefaults->load('403');
      }
      elseif ($system_config->get('page.404') === $entity_url) {
        $metatags = $this->metatagDefaults->load('403');
      }
    }