Hide block on 404 (not found) page

Question

SITUATION:

• A View page displays all "items" that have a certain "code".

• The code is passed via a contextual filter, e.g. /items/17, where 17 is the code.

• A user might try to guess a code by typing it in, e.g. /items/345.

• When the View is empty (i.e. the code is bogus), a not-found exception is raised as follows:

  function mymodule_views_pre_render(\Drupal\views\ViewExecutable $view) {
    if ($view->current_display == 'page_machine_name' && empty($view->result)) {
      throw new \Symfony\Component\HttpKernel\Exception\NotFoundHttpException();
    }
  }
  

• There is a menu included as a block that's displayed at the top of the View page.

• The menu block configuration restricts the block to pages with paths /items/*.

PROBLEM:

The menu also appears on the 404 page, since that page retains the bogus path /items/345. How can I exclude the menu block from the 404 page?

NOTES:

• I want to display the default 404 page rather than the view with a "not found" message as its body.
• I don't want to display the view as a block instead of as a page unless it's the only way. I already know how to solve the problem if I have to do that.
• If there were a way to say, "Include this block only on View pages" (like you can for content types), that would be ideal.
• There are actually three blocks that need to be excluded from the 404 page; I mention only one to make it simpler. The other blocks are a text block and a banner block.
• I see that a solution to this problem has been proposed for Drupal 9, but not available yet.

Answer 1

Yes, the solution is not available yet in core, see https://www.drupal.org/node/2245767.

But you can use this contrib module https://www.drupal.org/project/block_in_page_not_found and negate the condition to not show the block on a 404 page:

Answer 2

programmatically:

 $route_name = \Drupal::request()->attributes->get('_route');
  if ('system.404' === $route_name) {
    $region_name = 'content';
    $block_names = ['oneBlockID', 'andAnother'];
    foreach ($block_names as $block_name)
    if (isset($variables['page'][$region_name][$block_name])) {
      $variables['page'][$region_name][$block_name]['#access'] = FALSE;
    }
  }