Fix navbar handling

assets/js/navbar.js

@@ -2,6 +2,7 @@ const navbar = document.querySelector('.navbar')
 const toggler = document.getElementById('main-nav-toggler')
 const modeSelectors = document.querySelectorAll('.switch-mode-collapsed')
 
+if (navbar !== null && toggler !== null) {
   // set the navbar background color to opaque when scrolling past a breakpoint
   window.onscroll = () => {
     if (window.scrollY > 75) {
@@ -22,3 +23,4 @@ for (let i = 0; i < modeSelectors.length; ++i) {
       toggler.click()
     }
   }
+}