title: ngx-gallery not displaying image
body: <p>I am using <code>@ngx-gallery/lightbox</code> to display an image gallery. But it does not display any images. This I provide some code and <a href="https://stackblitz.com/edit/angular-qytkkw" rel="nofollow noreferrer">Stackblitz</a> demo as your reference</p>
  
  <p><strong>HTML</strong></p>
  
  <pre><code>&lt;button mat-button (click)="lightbox.open(0, 'lightbox')"&gt;Open Gallery&lt;/button&gt;
  </code></pre>
  
  <p><strong>Component</strong></p>
  
  <pre><code> items: GalleryItem[];
  
    constructor(public gallery: Gallery, public lightbox: Lightbox) {
  
    }
   ngOnInit() {
      // This is for Basic example
      this.items = imageData.map(item =&gt; {
        return new ImageItem(item.srcUrl, item.previewUrl);
      });
  
      // This is for Lightbox example
      this.gallery.ref('lightbox').load(this.items);
    }
  
  }
  </code></pre>