Indexmenu Plugin: If you use the 'js'-option of the indexmenu plugin, you have to disable the 'defer_js'-setting. This setting is temporary, in the future the indexmenu plugin will be improved.
If you are getting this error from the index menu, you need to disable defer_js. You can find it in the Site Settings section of the admin panel, under Feature Flags.
This is different from the standard template because it uses the bootstrap template, but the part about unchecking defer_js is the same.
I’ve read that this is a temporary fix, and that it will get better in an update, but as of September 8, 2020, there doesn’t seem to be an update yet.