Placing menu with dropdown anywhere buy Main navigation region triggers js error

I've created a multi-tiered menu. When I place it in the "Main navigation region", the dropdown functionality works, but if I place it in "Top navigation region" or any other block region, I get the following js error in the console:

dropdown.js:153 Uncaught TypeError: Bootstrap's dropdowns require Popper.js (https://popper.js.org/)
    at t.e.show (dropdown.js:153)
    at t.e.toggle (dropdown.js:126)
    at HTMLAnchorElement.<anonymous> (dropdown.js:367)
    at Function.each (jquery.min.js?v=3.5.1:2)
    at S.fn.init.each (jquery.min.js?v=3.5.1:2)
    at S.fn.init.t._jQueryInterface [as dropdown] (dropdown.js:353)
    at HTMLAnchorElement.<anonymous> (dropdown.js:519)
    at HTMLDocument.dispatch (jquery.min.js?v=3.5.1:2)
    at HTMLDocument.v.handle (jquery.min.js?v=3.5.1:2)
e.show @ dropdown.js:153
e.toggle @ dropdown.js:126
(anonymous) @ dropdown.js:367
each @ jquery.min.js?v=3.5.1:2
each @ jquery.min.js?v=3.5.1:2
t._jQueryInterface @ dropdown.js:353
(anonymous) @ dropdown.js:519
dispatch @ jquery.min.js?v=3.5.1:2
v.handle @ jquery.min.js?v=3.5.1:2

I modified the theme to include popper.js, but got a different js error:

Uncaught TypeError: a.default is not a constructor
    at t.e.show (dropdown.js:176)
    at t.e.toggle (dropdown.js:126)
    at HTMLAnchorElement.<anonymous> (dropdown.js:367)
    at Function.each (jquery.min.js?v=3.5.1:2)
    at S.fn.init.each (jquery.min.js?v=3.5.1:2)
    at S.fn.init.t._jQueryInterface [as dropdown] (dropdown.js:353)
    at HTMLAnchorElement.<anonymous> (dropdown.js:519)
    at HTMLDocument.dispatch (jquery.min.js?v=3.5.1:2)
    at HTMLDocument.v.handle (jquery.min.js?v=3.5.1:2)