Box machine name for Box view block should be validated

If you use Box view block, you must write machine name of box which you want show in the block. This machine name is not validated and if box with this machine name not exist, we get error:

The website encountered an unexpected error. Please try again later.
TypeError: Argument 1 passed to Drupal\Core\Entity\EntityViewBuilder::view() must implement interface Drupal\Core\Entity\EntityInterface, boolean given, called in /data/uniweb/drupal8/web/modules/contrib/box/src/Plugin/Block/BoxView.php on line 138 in Drupal\Core\Entity\EntityViewBuilder->view() (line 110 of core/lib/Drupal/Core/Entity/EntityViewBuilder.php).
Drupal\Core\Entity\EntityViewBuilder->view(, 'full') (Line: 138)
Drupal\box\Plugin\Block\BoxView->build() (Line: 203)
Drupal\block\BlockViewBuilder::preRender(Array)
call_user_func('Drupal\block\BlockViewBuilder::preRender', Array) (Line: 378)
Drupal\Core\Render\Renderer->doRender(Array) (Line: 450)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 490)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 81)
__TwigTemplate_f8a17f041581af9829850fd45db45c2b69d8b3a60b80408d416da5d23dbccb61->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('core/themes/seven/templates/page.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('page', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 490)
Drupal\Core\Template\TwigExtension->escapeFilter(Object, Array, 'html', NULL, 1) (Line: 90)
__TwigTemplate_be8c7bbb9c824f2826368d7c8da984c6279779db72a67fd8056a00bb23b816f2->doDisplay(Array, Array) (Line: 432)
Twig_Template->displayWithErrorHandling(Array, Array) (Line: 403)
Twig_Template->display(Array) (Line: 411)
Twig_Template->render(Array) (Line: 64)
twig_render_template('core/themes/classy/templates/layout/html.html.twig', Array) (Line: 384)
Drupal\Core\Theme\ThemeManager->render('html', Array) (Line: 437)
Drupal\Core\Render\Renderer->doRender(Array, ) (Line: 195)
Drupal\Core\Render\Renderer->render(Array) (Line: 147)
Drupal\Core\Render\MainContent\HtmlRenderer->Drupal\Core\Render\MainContent\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
Drupal\Core\Render\MainContent\HtmlRenderer->renderResponse(Array, Object, Object) (Line: 90)
Drupal\Core\EventSubscriber\MainContentViewSubscriber->onViewRenderArray(Object, 'kernel.view', Object) (Line: 108)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.view', Object) (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 657)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Machine name should be validated.