Using downgradeModule to create an Angular hybrid app - angularjs

Question

I want to upgrade my Angular JS application to use Angular (2+), and for this I want to use the downgradedModule:
app.module.js (Angular JS module)
import angular from 'angular';
import { environment } from '../environment'
import { downgradeModule } from '#angular/upgrade/static';
import { platformBrowserDynamic } from '#angular/platform-browser-dynamic';
import { AppModule } from './app/app.module';
const bootstrapFn = (extraProviders) => {
const platformRef = platformBrowserDynamic(extraProviders);
return platformRef.bootstrapModule(AppModule);
};
const downgradedModule = downgradeModule(bootstrapFn);
(function() {
'use strict';
angular
.module('app', [
/* my other Angular JS modules */
downgradedModule
])
..........
app.module.ts (Angular module)
import { BrowserModule } from '#angular/platform-browser';
import { NgModule } from '#angular/core';
import { UpgradeModule, setAngularLib } from '#angular/upgrade/static';
import { AppRoutingModule } from './app-routing.module';
import angular from 'angular';
import { MyComponent} from './example-components/my-component.component';
setAngularLib(angular);
#NgModule({
imports: [
BrowserModule,
UpgradeModule,
AppRoutingModule
],
declarations: [
MyComponent
],
entryComponents: [
MyComponent
]
})
export class AppModule {
constructor(private upgrade: UpgradeModule) { }
ngDoBootstrap() {}
}
When I run npm start, it says that angular is not defined and points to the angular.module('app',... line from the app.module.js file. If I go to the only page which contains Angular JS and also Angular components, I don't get any error, but the page is not displaying anything.
If I try to use UpgradeModule into the app.module.ts file like this:
ngDoBootstrap() {
this.upgrade.bootstrap(document.documentElement, ['app'], {
strictDi: false
});
}
The application is working but I get an error into the browser's console:
Error: [$rootScope:inprog] $digest already in progress. I understand that this can be avoided if I use downgradedModule which doesn't work for now as I explained.
Do I have to do something else to bootstrap the application, or what is the problem? Thanks!