Ionic cordova build ios - Erro no build do Ios

Olá, estou fazendo o curso de Ionic versão 3 e estou tendo problemas no build do Ionic para a plataforma IOS. O erro:

Error: Cannot read property 'toLowerCase' of undefined

[ERROR] An error occurred while running cordova build ios (exit code 1).      

Já procurei em inúmeros fóruns, mas não consigo achar a resposta ou algo que me ajude. Alguém consegue me ajudar? Por favor

Vou disponibilizar arquivos que acho importante para a resolução:

package.json

    "dependencies": {
        "@angular/common": "4.4.4",
        "@angular/compiler": "4.4.4",
        "@angular/compiler-cli": "4.4.4",
        "@angular/core": "4.4.4",
        "@angular/forms": "4.4.4",
        "@angular/http": "4.4.4",
        "@angular/platform-browser": "4.4.4",
        "@angular/platform-browser-dynamic": "4.4.4",
        "@ionic-native/camera": "^4.5.2",
        "@ionic-native/core": "4.3.2",
        "@ionic-native/date-picker": "^4.5.2",
        "@ionic-native/onesignal": "^4.5.2",
        "@ionic-native/splash-screen": "4.3.2",
        "@ionic-native/status-bar": "4.3.2",
        "@ionic-native/vibration": "^4.5.2",
        "@ionic/storage": "2.0.1",
        "cordova-android": "^6.2.3",
        "cordova-ios": "^4.4.0",
        "cordova-plugin-camera": "^2.4.1",
        "cordova-plugin-compat": "^1.2.0",
        "cordova-plugin-datepicker": "^0.9.3",
        "cordova-plugin-device": "^1.1.4",
        "cordova-plugin-ionic-webview": "^1.2.1",
        "cordova-plugin-splashscreen": "^4.0.3",
        "cordova-plugin-vibration": "^2.1.6",
        "cordova-plugin-whitelist": "^1.3.1",
        "ionic-angular": "3.8.0",
        "ionic-plugin-keyboard": "^2.2.1",
        "ionicons": "3.0.0",
        "rxjs": "5.4.3",
        "sw-toolbox": "3.6.0",
        "zone.js": "0.8.18"
    },
    "devDependencies": {
        "@ionic/app-scripts": "^3.2.4",
        "typescript": "2.3.4"
    },
    "description": "An Ionic project",
    "cordova": {
        "plugins": {
            "cordova-plugin-vibration": {},
            "cordova-plugin-datepicker": {},
            "cordova-plugin-device": {},
            "cordova-plugin-ionic-webview": {},
            "cordova-plugin-splashscreen": {},
            "cordova-plugin-whitelist": {},
            "ionic-plugin-keyboard": {},
            "cordova-plugin-camera": {
                "CAMERA_USAGE_DESCRIPTION": " ",
                "PHOTOLIBRARY_USAGE_DESCRIPTION": " "
            }
        },
        "platforms": [
            "android",
            "ios"
        ]
    }
}

app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { ErrorHandler, NgModule } from '@angular/core';
import { IonicApp, IonicErrorHandler, IonicModule } from 'ionic-angular';
import { SplashScreen } from '@ionic-native/splash-screen';
import { StatusBar } from '@ionic-native/status-bar';
import { MyApp } from './app.component';
import { HttpClientModule } from '@angular/common/http';
import { CarrosServiceProvider } from '../providers/carros-service/carros-service';
import { AgendamentosServiceProvider } from '../providers/agendamentos-service/agendamentos-service';

import 'rxjs/add/operator/finally';
import 'rxjs/add/operator/do';
import 'rxjs/add/operator/mergeMap';
import 'rxjs/add/observable/fromPromise';
import 'rxjs/add/observable/of';
import 'rxjs/add/operator/catch';

import { IonicStorageModule } from '@ionic/storage';
import { AgendamentoDaoProvider } from '../providers/agendamento-dao/agendamento-dao';
import { UsuariosServiceProvider } from '../providers/usuarios-service/usuarios-service';
import { UrlServiceProvider } from '../providers/url-service/url-service';
import { Vibration } from '@ionic-native/vibration';
import { DatePicker } from '@ionic-native/date-picker';
import { Camera } from '@ionic-native/camera';
//import { OneSignal } from '@ionic-native/onesignal'; 


@NgModule({
  declarations: [
    MyApp
  ],
  imports: [
    BrowserModule,
    HttpClientModule,
    IonicModule.forRoot(MyApp),
    IonicStorageModule.forRoot({  
      name: 'aluracar',           
      storeName: 'agendamentos',  
      driverOrder: ['indexeddb']  
    })
  ],
  providers: [
    StatusBar,
    SplashScreen,
    {provide: ErrorHandler, useClass: IonicErrorHandler},
    CarrosServiceProvider,
    AgendamentosServiceProvider,
    AgendamentoDaoProvider,
    UsuariosServiceProvider,
    UrlServiceProvider,
    Vibration,
    DatePicker,
    Camera,