After further investigation, it appears that a script that I had set to load in the JavaScript code section of the Theme Options was causing this issue.
This was the offending code. After removal, the page now loads correctly and the drop down menus work as expected.
COPY CODE
<!-- Start of Mailchimp popup form -->
<script type="text/javascript" src="//downloads.mailchimp.com/js/signup-forms/popup/embed.js" data-dojo-config="usePlainJson: true, isDebug: false"></script><script type="text/javascript">require(["mojo/signup-forms/Loader"], function(L) { L.start({"baseUrl":"mc.us17.list-manage.com","uuid":"e079d75547b7b32b5d36f5c9a","lid":"5a7483555a"}) })</script>
<!-- End of Mailchimp popup form -->