Why do I see the admin panel with access denied?

[AlexKomMsk]

In the config, I specified the middleware
App\Http\Middleware\CheckIfAdmin::class,

I try to go to the admin url with no admin user and see the edges of the admin panel on the background of the 404 page.

Why do I see the left admin menu? How to turn it off?

[AlexKomMsk]

Removed check in file
views.errors.layout

It is better to think over this point otherwise there may be security problems in the admin area.

[pxpm]

@AlexKomMsk what is the contents of App\Http\Middleware\CheckIfAdmin::class ?

If I return false from checkIfUserIsAdmin() I don't see any sidebar.

If you are talking about roles and permissions, you should add your checks directly in the sidebar file:

@if(backpack_user()->hasRole('admin'))
//show items only available for users with admin role
@endif

Also you should add those permission checks in your controllers so that even if someone try direct access to the resource url, the controller would deny the access.
You can control that access with $this->crud->allowAccess(['list','create']) and denyAccess-> in your controllers.

Cheers