Render context is empty

When we inject the toolbar loader code at the end of body in WebprofilerEventSubscriber we directly use the render() function, but it doesn't work anymore because of #2450993: Rendered Cache Metadata created during the main controller request gets lost:

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

LogicException: Render context is empty, because render() was called outside of a renderRoot() or renderPlain() call. Use renderPlain()/renderRoot() or #lazy_builder/#pre_render instead. in Drupal\Core\Render\Renderer->doRender() (line 229 of core/lib/Drupal/Core/Render/Renderer.php).

Drupal\Core\Render\Renderer->render(Array)
render(Array)
Drupal\webprofiler\EventSubscriber\WebprofilerEventSubscriber->injectToolbar(Object)
Drupal\webprofiler\EventSubscriber\WebprofilerEventSubscriber->onKernelResponse(Object, 'kernel.response', Object)
call_user_func(Array, Object, 'kernel.response', Object)
Symfony\Component\EventDispatcher\Debug\WrappedListener->__invoke(Object, 'kernel.response', Object)
Drupal\Component\EventDispatcher\ContainerAwareEventDispatcher->dispatch('kernel.response', Object)
Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcher->dispatch('kernel.response', Object)
Symfony\Component\HttpKernel\HttpKernel->filterResponse(Object, Object, 1)
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\webprofiler\StackMiddleware\WebprofilerMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)