How to test a middleware with phpunit in laravel 5.5?

How could I test my middleware ? Here I am testing if a admin can access a middleware protected route, that returns a 500, if the user does not have a privileged ip - then the middleware returns a 401 (not authorized) when trying to access the /500 page .

My test:

use App\Http\Middleware\OnlyAdminIp;
use Illuminate\Http\Request;
use Tests\TestCase;
use Illuminate\Foundation\Testing\DatabaseTransactions;

class HttpTests extends TestCase
{
    use DatabaseTransactions;

   /** @test */
    public function if_a_500_page_returns_a_500_response_for_admin()
    {
        $request = Request::create(config('app.url') . '500', 'GET');
        $middleware = new OnlyAdminIp();
        $response = $middleware->handle($request, function () {});
        $this->assertEquals($response->getStatusCode(), 401);
    }
}

My middleware:

namespace App\Http\Middleware;

use App\IpList;
use Closure;

class OnlyAdminIp
{
    /**
     * Handle an incoming request.
     *
     * @param  \Illuminate\Http\Request $request
     * @param  \Closure $next
     * @return mixed
     */
    public function handle($request, Closure $next)
    {
        $client_ip = $_SERVER["HTTP_CF_CONNECTING_IP"] ?? $request->ip(); // CDN(Cloudflare) provides the real client ip, a safeguard is used to prevent critical error if CDN is removed/changed.
        $ipList = IpList::all()
            ->pluck('ip')
            ->toArray();
        if (!in_array($client_ip, $ipList)) {
            abort(401);
        }

        return $next($request);
    }
}

And just for more clarity - the 500 route (in web.php) .

Route::group(['middleware' => 'admin.ip'], function () {


    Route::get('500', function () {
        abort(500);
    }); 

});

With this setup I am getting Call to a member function getStatusCode() on null Thanks in advance !