Make NullGenerator fail gracefully, since we can't yet avoid it being called at all.

core/lib/Drupal/Core/Routing/NullGenerator.php

@@ -8,6 +8,7 @@
 namespace Drupal\Core\Routing;
 use Symfony\Component\Routing\Generator\UrlGeneratorInterface;
 use Symfony\Component\Routing\RequestContext;
+use Symfony\Component\Routing\Exception\RouteNotFoundException;
 
 /**
  * No-op implementation of a Url Generator, needed for backward compatibility.
@@ -15,14 +16,13 @@
 class NullGenerator implements UrlGeneratorInterface {
 
   public function generate($name, $parameters = array(), $absolute = FALSE) {
-    throw new \Exception('Method disabled. This is a dummy implementation.');
+    return '';
+    throw new RouteNotFoundException();
   }
 
   public function setContext(RequestContext $context) {
-    throw new \Exception('Method disabled. This is a dummy implementation.');
   }
 
   public function getContext() {
-    throw new \Exception('Method disabled. This is a dummy implementation.');
   }
 }