Hi, it’s not because of “popper.js”, it’s because you are using an old version of the SmartMenus Bootstrap addon which is for Bootstrap 3 but at the same time you are using Bootstrap 4. So you would need to do a few things to cope properly with the issue:
1) Download the latest SmartMenus release and upgrade all your files:
with the latest versions (and make sure you are using “jquery.smartmenus.bootstrap-4.js” and “jquery.smartmenus.bootstrap-4.css” instead of “jquery.smartmenus.bootstrap.js” and “jquery.smartmenus.bootstrap.css”).
2) You are also using some custom classes in your HTML menu structure so you would need to change them to avoid some issues (like the menu not being initialized at all, duplicate sub menu indicator arrows, etc.)
a) Add the “navbar-nav” class to your root UL element: