Issue #2795395 by Yogesh Pawar, tresti88, marvin_B8, shashikant_chauhan, tstoeckler, dawehner: PathRootsSubscriber breaks if there are no routes

core/lib/Drupal/Core/EventSubscriber/PathRootsSubscriber.php

@@ -20,7 +20,7 @@ class PathRootsSubscriber implements EventSubscriberInterface {
    *
    * @var array
    */
-  protected $pathRoots;
+  protected $pathRoots = [];
 
   /**
    * The state key value store.
@@ -58,7 +58,7 @@ class PathRootsSubscriber implements EventSubscriberInterface {
    */
   public function onRouteFinished() {
     $this->state->set('router.path_roots', array_keys($this->pathRoots));
-    unset($this->pathRoots);
+    $this->pathRoots = [];
   }
 
   /**

core/tests/Drupal/Tests/Core/EventSubscriber/PathRootsSubscriberTest.php

@@ -43,6 +43,11 @@ class PathRootsSubscriberTest extends UnitTestCase {
    * @covers ::onRouteFinished
    */
   public function testSubscribing() {
+
+    // Ensure that onRouteFinished can be called without throwing notices
+    // when no path roots got set.
+    $this->pathRootsSubscriber->onRouteFinished();
+
     $route_collection = new RouteCollection();
     $route_collection->add('test_route1', new Route('/test/bar'));
     $route_collection->add('test_route2', new Route('/test/baz'));