Sorry, I don’t know what is causing this.
frontend.js is not part of PhotoSwipe, my plugin uses lightbox-photoswipe/lib/photoswipe.min.js, lightbox-photoswipe/lib/photoswipe-ui-default.min.js and lightbox-photoswipe/js/photoswipe.min.js.
The site also has a number of other problems as well. When opening https://www.woodenhottubsale.co.uk/product/iglu-garden-sauna/ I get the following errors in the JS console:
ReferenceError: jQuery is not defined — iglu-garden-sauna:6:1
ReferenceError: jQuery is not defined — iglu-garden-sauna:14:1
ReferenceError: jQuery is not defined — iglu-garden-sauna:57:1
ReferenceError: jQuery is not defined — iglu-garden-sauna:183:1
ReferenceError: jQuery is not defined — iglu-garden-sauna:187:1
Furthermore the HTML code is also not valid, there are many errors, which should be fixed:
https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.woodenhottubsale.co.uk%2Fproduct%2Figlu-garden-sauna%2F
So I assume the errors with PhotoSwipe are caused by some of your other plugins or the theme you use – but this is way out of scope for me to check all the errors you have on your site, sorry.
