Issue #1986888 by tim.plunkett, agentrickard: Fixed Field UI integration for entity types is brittle.

core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php

@@ -49,6 +49,7 @@ class RouteSubscriber implements EventSubscriberInterface {
   public function routes(RouteBuildEvent $event) {
     $collection = $event->getRouteCollection();
     foreach ($this->manager->getDefinitions() as $entity_type => $entity_info) {
+      $defaults = array();
       if ($entity_info['fieldable'] && isset($entity_info['route_base_path'])) {
         $path = $entity_info['route_base_path'];
 

core/modules/field_ui/lib/Drupal/field_ui/Tests/FieldUIRouteTest.php

@@ -0,0 +1,55 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\field_ui\Tests\FieldUIRouteTest.
+ */
+
+namespace Drupal\field_ui\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests the functionality of the Field UI route subscriber.
+ */
+class FieldUIRouteTest extends WebTestBase {
+
+  /**
+   * Modules to enable.
+   */
+  public static $modules = array('field_ui_test', 'node');
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function getInfo() {
+    return array(
+      'name' => 'Field UI routes',
+      'description' => 'Tests the functionality of the Field UI route subscriber.',
+      'group' => 'Field UI',
+    );
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    $this->drupalCreateContentType(array('type' => 'article', 'name' => 'Article'));
+
+    $this->drupalLogin($this->root_user);
+  }
+
+  /**
+   * Ensures that entity types with bundles do not break following entity types.
+   */
+  public function testFieldUIRoutes() {
+    $this->drupalGet('field-ui-test-no-bundle/manage/fields');
+    $this->assertText('No fields are present yet.');
+
+    $this->drupalGet('admin/structure/types/manage/article/fields');
+    $this->assertTitle('Article | Drupal');
+  }
+
+}

core/modules/field_ui/tests/modules/field_ui_test/field_ui_test.info.yml

@@ -0,0 +1,10 @@
+name: "Field UI tests"
+type: module
+description: "Support module for Field UI testing."
+package: Testing
+version: VERSION
+core: 8.x
+hidden: TRUE
+
+dependencies:
+  - field_ui

core/modules/field_ui/tests/modules/field_ui_test/field_ui_test.module

@@ -0,0 +1,6 @@
+<?php
+
+/**
+ * @file
+ * Field UI test module.
+ */

core/modules/field_ui/tests/modules/field_ui_test/lib/Drupal/field_ui_test/Plugin/Core/Entity/FieldUITestNoBundle.php

@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\field_ui_test\Plugin\Core\Entity\FieldUITestNoBundle.
+ */
+
+namespace Drupal\field_ui_test\Plugin\Core\Entity;
+
+use Drupal\Core\Entity\Entity;
+use Drupal\Core\Entity\Annotation\EntityType;
+use Drupal\Core\Annotation\Translation;
+
+/**
+ * Defines the test Field UI class.
+ *
+ * @EntityType(
+ *   id = "field_ui_test_no_bundle",
+ *   label = @Translation("Test Field UI entity, no bundle"),
+ *   module = "field_ui_test",
+ *   controllers = {
+ *     "storage" = "Drupal\Core\Entity\DatabaseStorageController"
+ *   },
+ *   fieldable = TRUE,
+ *   route_base_path = "field-ui-test-no-bundle/manage"
+ * )
+ */
+class FieldUITestNoBundle extends Entity {
+
+  /**
+   * The entity ID.
+   *
+   * @var int
+   */
+  public $id;
+
+}