Unable to delete a field_group

yes, this bug is reproducible, will look into this.

complete log about this error.

The website encountered an unexpected error. Please try again later.

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 130 of core/lib/Drupal/Core/Entity/EntityTypeManager.php).

Drupal\Core\Entity\EntityManager->getDefinition(NULL)
Drupal\field_ui\Routing\FieldUiRouteEnhancer->enhance(Array, Object)
Drupal\Core\Routing\LazyRouteEnhancer->enhance(Array, Object)
Symfony\Cmf\Component\Routing\DynamicRouter->applyRouteEnhancers(Array, Object)
Symfony\Cmf\Component\Routing\DynamicRouter->matchRequest(Object)
Symfony\Cmf\Component\Routing\ChainRouter->doMatch('/admin/structure/types/manage/article/groups/node.article.form.default.group_new_fielset/delete', Object)
Symfony\Cmf\Component\Routing\ChainRouter->matchRequest(Object)
Drupal\Core\Routing\AccessAwareRouter->matchRequest(Object)
Symfony\Component\HttpKernel\EventListener\RouterListener->onKernelRequest(Object, 'kernel.request', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.request', Object)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)

Fix is ready, committing it in an hour.

Should be fixed