diff -u b/core/includes/menu.inc b/core/includes/menu.inc
--- b/core/includes/menu.inc
+++ b/core/includes/menu.inc
@@ -2740,6 +2740,9 @@
   }
 
   // Find any item whose router path does not exist any more.
+  // Do not delete entries with an empty path as this can remove menu links in
+  // the process of being created.
+  $paths[] = '';
   $query = \Drupal::entityQuery('menu_link')
     ->condition('router_path', $paths, 'NOT IN')
     ->condition('external', 0)
reverted:
--- b/core/profiles/standard/standard.install
+++ a/core/profiles/standard/standard.install
@@ -51,7 +51,6 @@
     'link_path' => '<front>',
     'menu_name' => 'main',
   ));
-  menu_router_rebuild();
   $menu_link->save();
 
   // Enable the Contact link in the footer menu.
only in patch2:
unchanged:
--- a/core/modules/menu_link/lib/Drupal/menu_link/Entity/MenuLink.php
+++ b/core/modules/menu_link/lib/Drupal/menu_link/Entity/MenuLink.php
@@ -542,7 +542,7 @@ public function preSave(EntityStorageControllerInterface $storage_controller) {
     }
     // Find the route_name.
     if (!isset($this->route_name)) {
-      if ($result = \Drupal::service('router.matcher.final_matcher')->findRouteNameParameters($this->link_path)) {
+      if (!$this->external && $result = \Drupal::service('router.matcher.final_matcher')->findRouteNameParameters($this->link_path)) {
         list($this->route_name, $this->route_parameters) = $result;
       }
       else {