Fetch All Data in back4app class ionic

Question

I'm new with Ionic and i want to make a CRUD ionic app with Back4app. I managed to create some class and the datas within it, using the app, but i have no idea how to Read (all data) and put it in a List within my app. Can someone help me?

this is my ts file

export class UnitFormPage {

voucherList: VoucherModel[] = []

 constructor(public navCtrl: NavController, public navParams: NavParams) {
}

private readAllData(){

let Vouchers = Parse.Object.extend("Voucher");
let query = new Parse.Query(Vouchers);

query.find().then((result)=>{
       this.voucherList = result //this List contains null, but in console it did fetch the data
  if (typeof document !== 'undefined')
    // document.write(`Voucher found: ${JSON.stringify(result)}`);
  console.log('Voucher found', result);
}, (error) => {
    // if (typeof document !== 'undefined') document.write(`Error while fetching Voucher: ${JSON.stringify(error)}`);
    console.error('Error while fetching Voucher', error);
})

}

my model class

export class VoucherModel{
public id: number = 0;
public unitRequestor: string = "";
public picRequestor: string = "";
public unitTujuan: string = "";
public jenisPermintaan: string = "";
public deskripsiPermintaan: string = "";
public tglPermintaan: string = "";
public jamPermintaan: string = "";

}

this is part of my view class

<ion-item-sliding *ngFor="let item of voucherList" >
<ion-item >
  <ion-grid item-end no-padding>
    <ion-row>
      <ion-col >
        <ion-label style="font-size: 20px; font-weight: bold;">{{item.unitRequestor}}</ion-label>
      </ion-col>
    </ion-row>

the object did show in console but not in app. Any help appreciated.