FirebaseApp.initializeApp(Context) error in ionic 4 with angular

I'm totally new to ionic and angular. I'm getting an issue in my ionic project. I have installed Angular firebase and firebase plugins. Error:

Default FirebaseApp is not initialized in this process com.xxx.xxx Make sure to call FirebaseApp.initializeApp(Context) first.

Firestore operations are working properly. But the only problem is when i'm trying to get the device fcm token it's throwing the error. Here is my code

app.module.ts

import { NgModule , AfterViewInit } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
import { IonicModule, IonicRouteStrategy , IonSlides } from '@ionic/angular';
import { SplashScreen } from '@ionic-native/splash-screen/ngx';
import { StatusBar } from '@ionic-native/status-bar/ngx';

import { AppRoutingModule } from './app-routing.module';
import { AppComponent } from './app.component';
import { AngularFirestore , AngularFirestoreModule } from '@angular/fire/firestore';
import { AngularFireModule } from '@angular/fire';
import { Firebase } from '@ionic-native/firebase/ngx';
import { FcmService } from './services/notification/fcm.service';


@NgModule({
  declarations: [AppComponent],
  entryComponents: [],
  imports: [
    CommonModule,
    BrowserModule,
    IonicModule.forRoot(), 
    AppRoutingModule, 
    IonicStorageModule.forRoot(),
    HttpClientModule,
    FormsModule,
    ReactiveFormsModule,
    AngularFireModule.initializeApp(environment.firebaseconfig), 
    AngularFirestoreModule
  ],
  schemas: [ CUSTOM_ELEMENTS_SCHEMA],
  providers: [
    StatusBar,
    SplashScreen,
    IonSlides,
    { provide: RouteReuseStrategy, useClass: IonicRouteStrategy },
    AngularFirestore,
    FcmService,
    Firebase
  ],
  bootstrap: [AppComponent]
})
export class AppModule {

}

app.component.ts

import { Component } from '@angular/core';
import { Platform, Events } from '@ionic/angular';
import { SplashScreen } from '@ionic-native/splash-screen/ngx';
import { StatusBar } from '@ionic-native/status-bar/ngx';
import { FcmService } from './services/notification/fcm.service';


@Component({
  selector: 'app-root',
  templateUrl: 'app.component.html'
})
export class AppComponent {

  constructor(
    private platform: Platform,
    private splashScreen: SplashScreen,
    private statusBar: StatusBar,
    private fcm: FcmService,
  ) {
    this.initializeApp();
  }

  private notificationSetup() {
    this.fcm.getToken();
    this.fcm.onNotifications().subscribe(
      (msg) => {
        if (this.platform.is('ios')) {
          this.alertSerivce.presentToast(msg.aps.alert);
        } else {
          this.alertSerivce.presentToast(msg.body);
        }
      });
  }

  initializeApp() {
    this.platform.ready().then(() => {
      this.statusBar.styleDefault();
      this.splashScreen.hide();
      this.notificationSetup();
    });
  }

}

fcm.service.ts

import { Injectable } from '@angular/core';
import { AngularFirestore } from '@angular/fire/firestore';
import { Firebase } from '@ionic-native/firebase/ngx';
import { Platform } from '@ionic/angular';



@Injectable({
  providedIn: 'root'
})
export class FcmService {

  constructor(
    private firebase: Firebase,
    private afs: AngularFirestore,
    private platform: Platform,
  ) { }

  async getToken() {
    let token: string;

    if (this.platform.is('android')) {
       await this.firebase.getToken().then((fcmtoken) => {
        token = fcmtoken
        console.log('Permission granted! Save to the server!', fcmtoken);
      }).catch((err) => {
        console.log('Error while getting fcm token', err)
      })
    }

    if (this.platform.is('ios')) {
      await this.firebase.getToken().then((fcmtoken) => {
        token = fcmtoken
        console.log('Permission granted! Save to the server!', fcmtoken);
      }).catch((err) => {
        console.log('Error while getting fcm token', err)
      })
      await this.firebase.grantPermission();
    }
    this.alert.presentToast(`FCMToken: ${token}`)
    this.saveToken(token);
    return token
  }

  private saveToken(token) {
    if (!token) return;
    this.storage.set(constants.LocalStorageKeys.fcmToken, token)
    return token
  }

  onNotifications() {
    return this.firebase.onNotificationOpen();
  }
}

package.json

 "dependencies": {
    "@angular/common": "^7.2.2",
    "@angular/core": "^7.2.2",
    "@angular/fire": "^5.1.2",
    "@ionic-native/core": "^5.0.0",
    "@ionic-native/firebase": "^5.0.0",
     ...
  }

I'm i missing anything?. I have been stuck in this issue from 2 days. I have followed this https://medium.freecodecamp.org/how-to-get-push-notifications-working-with-ionic-4-and-firebase-ad87cc92394e