silverstripe-subsites/_config/middleware.yml

---
Name: subsitesmiddleware
After:
  - requestprocessors
---
SilverStripe\Core\Injector\Injector:
  SilverStripe\Control\Director:
    properties:
      Middlewares:
        SubsitesStateMiddleware: %$SilverStripe\Subsites\Middleware\InitStateMiddleware