Hey. Sorry missed out to see this comment Sarah. Although this could be due to quite lot of reasons, my best guess is that the page contains other embedded items apart from the Google Slides (or even ads, youtube, etc).

If that’s the case try replacing the first line of the first bookmark from:
document.getElementsByTagName(“iframe”)[0];

to this:
document.querySelectorAll(‘iframe[src*=docs\\.google]’)[0];