Laravel method in getCustomAttribute returning cullection null - laravel

Question

I am trying to modify $category->slug by getSlugAttribute(). My Category class like this,
<?php
namespace App;
use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\Collection;
class Category extends Model
{
public function getSlugAttribute(){
return $this->getRoute($this);
}
//Testing _____________________________________________________
private $routes = [];
public function getRoute(Category $category)
{
$this->determineCategoriesRoutes();
return $this->routes[$category->id];
}
private function determineCategoriesRoutes()
{
$categories = $this->all()->keyBy('id');
foreach ($categories as $id => $category) {
$slugs = $this->determineCategorySlugs($category, $categories);
if (count($slugs) === 1) {
$this->routes[$id] = '/' . $slugs[0];
}
else {
$this->routes[$id] = '/' . implode('/', $slugs);
}
}
}
private function determineCategorySlugs(Category $category, Collection $categories, array $slugs = [])
{
array_unshift($slugs, $category->slug);
if ($category->parent_id != 0) {
$slugs = $this->determineCategorySlugs($categories[$category->parent_id], $categories, $slugs);
}
return $slugs;
}
//End Testing__________________________________________________
//end of this class
}
Now when I request for categories it's just loading and giving me a blank page.
Route::get('/test', function(App\Category $category){
$categories = $category->all();
return $categories;
});
This is my test route, also there is no error.