Cannot read properties of undefined (reading ‘firstChild’) using photoswipe in product page

I was doing it wrong. Data has to go into javascript, and orignal html should not be changed

<div class="pswp" tabindex="-1" role="dialog" aria-hidden="true">
                <div class="pswp__bg"></div>

                
                <div class="pswp__scroll-wrap">

                    <!-- Container that holds slides. -->
                    <div class="pswp__container">
                        <!-- don't modify these 3 pswp__item elements, data is added later on -->
                        <div class="pswp__item"></div>
                        <div class="pswp__item"></div>
                        
                    </div>

                    <!-- Default -->
                    <div class="pswp__ui pswp__ui--hidden">

                        <div class="pswp__top-bar">

                            <!--  Controls are self-explanatory. Order can be changed. -->

                            <div class="pswp__counter"></div>

                            <button class="pswp__button pswp__button--close" title="Close (Esc)"></button>
<button class="pswp__button pswp__button--share" title="Share"></button>

                            <button class="pswp__button pswp__button--fs" title="Toggle fullscreen"></button>

                            <button class="pswp__button pswp__button--zoom" title="Zoom in/out"></button>

                            
                            <div class="pswp__preloader">
                                <div class="pswp__preloader__icn">
                                    <div class="pswp__preloader__cut">
                                        <div class="pswp__preloader__donut"></div>
                                    </div>
                                </div>
                            </div>
                        </div>

                        <div class="pswp__share-modal pswp__share-modal--hidden pswp__single-tap">
                            <div class="pswp__share-tooltip"></div>
                        </div>

                        <button class="pswp__button pswp__button--arrow--left" title="Previous (arrow left)">
                        </button>

                        <button class="pswp__button pswp__button--arrow--right" title="Next (arrow right)">
                        </button>

                        <div class="pswp__caption">
                            <div class="pswp__caption__center"></div>
                        </div>

                    </div>

                </div>

        </div>
 <script src="https://cdnjs.cloudflare.com/ajax/libs/photoswipe/4.1.2/photoswipe.min.js"/></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/photoswipe/4.1.2/photoswipe-ui-default.min.js"/></script>       
<script type="text/javascript" >
            var openPhotoSwipe = function() {
                var pswpElement = document.querySelectorAll('.pswp')[0];

                // build items array
                var items = [
                {
                    html: '<iframe src="http://www.africau.edu/images/default/sample.pdf"></iframe>'
                },
                {
                    html: '<iframe src="http://www.africau.edu/images/default/sample.pdf"></iframe>'
                    
                }
                ];

                // define options (if needed)
                var options = {
                    // history & focus options are disabled on CodePen        
                    history: false,
                    focus: false,

                    showAnimationDuration: 0,
                    hideAnimationDuration: 0

                };

                var gallery = new PhotoSwipe( pswpElement, PhotoSwipeUI_Default, items, options);
                gallery.init();
            };

            //openPhotoSwipe();

            document.getElementById('btnShow').onclick = openPhotoSwipe;
        </script>