__WEBPACK_IMPORTED_MODULE_1_firebase_app__.app is not a function

Question

Firebase provider

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { AngularFireDatabase, FirebaseListObservable } from 'angularfire2/database';

@Injectable() export class FirebaseProvider {
  constructor(public afd: AngularFireDatabase) { }

  getFoodItems() {
    return this.afd.list("/foodItems");
  }

  addFood(foodName) {
    this.afd.list("/foodItems").push(foodName);
  }

  removeFood(id) {
    this.afd.list("/foodItems").remove(id);
  }
}

Here I am using Firebase provider (predefined methods)

import { Component } from '@angular/core';
import { NavController } from 'ionic-angular';
import { StepperPage } from '../stepper/stepper';
import { AboutUsPage } from '../about-us/about-us';
import { FirebaseProvider } from '../../providers/firebase/firebase';
import { FirebaseListObservable } from 'angularfire2/database';

@Component({
  selector: 'page-home',
  templateUrl: 'home.html'
})
export class HomePage {
  foodItems: FirebaseListObservable<any[]>;
  newItem = '';

  constructor(
    public navCtrl: NavController,
    public firebaseProvider: FirebaseProvider
  ) {
    this.foodItems = this.firebaseProvider.getFoodItems();
  }

  addFood() {
    this.firebaseProvider.addFood(this.newItem);
  }

  removeFood(id) {
    this.firebaseProvider.removeFood(id);
  }
}

Error I am getting

ERROR Error: Uncaught (in promise): TypeError: WEBPACK_IMPORTED_MODULE_1_firebase_app.app is not a function TypeError: WEBPACK_IMPORTED_MODULE_1_firebase_app.app is not a function