fix(core): unsubscribe from the `onError` when the root view is removed (#39940)

At the moment, when creating a root module, a subscription to the
`onError` subject is also created. It captures the scope where `NgModuleRef`
is created and prevents it from being garbage collected. Also note that this
`NgModuleRef` has a reference to the root module instance (e.g. `AppModule`),
which also prevents it from being GC'd.

PR Close #39940

packages/core/src/application_ref.ts

@@ -350,12 +350,17 @@ export class PlatformRef {
       if (!exceptionHandler) {
         throw new Error('No ErrorHandler. Is platform module (BrowserModule) included?');
       }
-      moduleRef.onDestroy(() => remove(this._modules, moduleRef));
+      ngZone!.runOutsideAngular(() => {
-      ngZone!.runOutsideAngular(() => ngZone!.onError.subscribe({
+        const subscription = ngZone!.onError.subscribe({
-        next: (error: any) => {
+          next: (error: any) => {
-          exceptionHandler.handleError(error);
+            exceptionHandler.handleError(error);
-        }
+          }
-      }));
+        });
+        moduleRef.onDestroy(() => {
+          remove(this._modules, moduleRef);
+          subscription.unsubscribe();
+        });
+      });
       return _callAndReportToErrorHandler(exceptionHandler, ngZone!, () => {
         const initStatus: ApplicationInitStatus = moduleRef.injector.get(ApplicationInitStatus);
         initStatus.runInitializers();

packages/core/test/acceptance/bootstrap_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {ApplicationRef, COMPILER_OPTIONS, Component, destroyPlatform, NgModule, TestabilityRegistry, ViewEncapsulation} from '@angular/core';
+import {ApplicationRef, COMPILER_OPTIONS, Component, destroyPlatform, NgModule, NgZone, TestabilityRegistry, ViewEncapsulation} from '@angular/core';
 import {expect} from '@angular/core/testing/src/testing_internal';
 import {BrowserModule} from '@angular/platform-browser';
 import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
@@ -227,6 +227,22 @@ describe('bootstrap', () => {
          }));
     });
 
+    describe('PlatformRef cleanup', () => {
+      it('should unsubscribe from `onError` when Injector is destroyed',
+         withBody('<my-app></my-app>', async () => {
+           const TestModule = createComponentAndModule();
+
+           const ngModuleRef = await platformBrowserDynamic().bootstrapModule(TestModule);
+           const ngZone = ngModuleRef.injector.get(NgZone);
+
+           expect(ngZone.onError.observers.length).toBe(1);
+
+           ngModuleRef.destroy();
+
+           expect(ngZone.onError.observers.length).toBe(0);
+         }));
+    });
+
     onlyInIvy('options cannot be changed in Ivy').describe('changing bootstrap options', () => {
       beforeEach(() => {
         spyOn(console, 'error');
@@ -365,4 +381,4 @@ export class MultipleSelectorsAppComponent {
   bootstrap: [MultipleSelectorsAppComponent],
 })
 export class MultipleSelectorsAppModule {
-}
+}