[BUGFIX beta] Avoid rerendering outlet state during router destruction.

During each `Ember.Route`'s `willDestroy` we trigger a
`run.once(this.router, '_setOutlets');`. This is so that the routes
views are destroyed properly (by removing them from the outlet state).

During `Ember.Router`'s `willDestroy` we clear
`this._toplevelView` (along with a bunch of other cleanup).

These two things combined can mean that `this._toplevelView` is `null`
when `_setOutlets` is called again (during the `Router`'s destruction).
In that scenario we are actually creating another
`this._toplevelView` and setting up another rendered root (since one
doesn't exist). When this happens the newly created root is never
cleaned up, since the Router's `willDestroy` has already ran and can no
longer clean up after itself.

packages/ember-routing/lib/system/router.js

@@ -140,6 +140,10 @@ const EmberRouter = EmberObject.extend(Evented, {
       this._engineInstances = new EmptyObject();
       this._engineInfoByRoute = new EmptyObject();
     }
+
+    // avoid shaping issues with checks during `_setOutlets`
+    this.isDestroyed = false;
+    this.isDestroying = false;
   },
 
   /*
@@ -266,6 +270,11 @@ const EmberRouter = EmberObject.extend(Evented, {
   },
 
   _setOutlets() {
+    // This is triggered async during Ember.Route#willDestroy.
+    // If the router is also being destroyed we do not want to
+    // to create another this._toplevelView (and leak the renderer)
+    if (this.isDestroying || this.isDestroyed) { return; }
+
     let handlerInfos = this.router.currentHandlerInfos;
     let route;
     let defaultParentState;

packages/ember/tests/application_lifecycle_test.js

@@ -125,6 +125,34 @@ QUnit.test('Destroying the application resets the router before the appInstance
   equal(controllerFor(appInstance, 'application').get('selectedMenuItem'), null);
 });
 
+QUnit.test('Destroying a route after the router does create an undestroyed `toplevelView`', function() {
+  App.Router.map(function() {
+    this.route('home', { path: '/' });
+  });
+
+  setTemplates({
+    index: compile('Index!'),
+    application: compile('Application! {{outlet}}')
+  });
+
+  App.IndexRoute = Route.extend();
+  run(App, 'advanceReadiness');
+
+  handleURL('/');
+
+  let router = appInstance.lookup('router:main');
+  let route = appInstance.lookup('route:index');
+
+  run(router, 'destroy');
+  equal(router._toplevelView, null, 'the toplevelView was cleared');
+
+  run(route, 'destroy');
+  equal(router._toplevelView, null, 'the toplevelView was not reinitialized');
+
+  run(App, 'destroy');
+  equal(router._toplevelView, null, 'the toplevelView was not reinitialized');
+});
+
 QUnit.test('initializers can augment an applications customEvents hash', function(assert) {
   assert.expect(1);
 

packages/ember/tests/routing/basic_test.js

@@ -3742,4 +3742,48 @@ if (isEnabled('ember-application-engines')) {
 
     throws(() => router.transitionTo('blog.post'), /Defining a custom serialize method on an Engine route is not supported/);
   });
+
+  QUnit.test('App.destroy does not leave undestroyed views after clearing engines', function() {
+    expect(4);
+
+    let engineInstance;
+    // Register engine
+    let BlogEngine = Engine.extend();
+    registry.register('engine:blog', BlogEngine);
+    let EngineIndexRoute = Route.extend({
+      init() {
+        this._super(...arguments);
+        engineInstance = getOwner(this);
+      }
+    });
+
+    // Register engine route map
+    let BlogMap = function() {
+      this.route('post');
+    };
+    registry.register('route-map:blog', BlogMap);
+
+    Router.map(function() {
+      this.mount('blog');
+    });
+
+    bootApplication();
+
+    let engine = container.lookup('engine:blog');
+    engine.register('route:index', EngineIndexRoute);
+    engine.register('template:index', compile('Engine Post!'));
+
+    handleURL('/blog');
+
+    let route = engineInstance.lookup('route:index');
+
+    run(router, 'destroy');
+    equal(router._toplevelView, null, 'the toplevelView was cleared');
+
+    run(route, 'destroy');
+    equal(router._toplevelView, null, 'the toplevelView was not reinitialized');
+
+    run(App, 'destroy');
+    equal(router._toplevelView, null, 'the toplevelView was not reinitialized');
+  });
 }