Cannot read property 'length' of undefined

[realmar]

I'm using your libraries as follow:

import {Component, OnInit} from '@angular/core';
import {UnityLoader} from 'unity-loader/UnityLoader';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.css']
})
export class AppComponent implements OnInit {

  unityLoader: UnityLoader;

  ngOnInit(): void {
    this.unityLoader = UnityLoader.instantiate("gameContainer", "assets/build.json", {onProgress: this.UnityProgress});
  }

  private UnityProgress(): void { }
}

However I got following error:

webpack-internal:///../../../core/esm5/core.js:1623 ERROR TypeError: Cannot read property 'length' of undefined
    at XMLHttpRequest.xhr.onload [as __zone_symbol__ON_PROPERTYload] (webpack-internal:///../../../../unity-loader/UnityLoader.js:225)
    at XMLHttpRequest.wrapFn (webpack-internal:///../../../../zone.js/dist/zone.js:1166)
    at ZoneDelegate.invokeTask (webpack-internal:///../../../../zone.js/dist/zone.js:425)
    at Object.onInvokeTask (webpack-internal:///../../../core/esm5/core.js:4940)
    at ZoneDelegate.invokeTask (webpack-internal:///../../../../zone.js/dist/zone.js:424)
    at Zone.runTask (webpack-internal:///../../../../zone.js/dist/zone.js:192)
    at ZoneTask.invokeTask [as invoke] (webpack-internal:///../../../../zone.js/dist/zone.js:499)
    at invokeTask (webpack-internal:///../../../../zone.js/dist/zone.js:1540)
    at XMLHttpRequest.globalZoneAwareCallback (webpack-internal:///../../../../zone.js/dist/zone.js:1566)

What am I doing wrong?