refactor: improve the efficiency of GLightbox switching

Repeatedly using `GLightbox.destroy()` (>= 4 times) will cause `GLightbox` to fail to generate new objects

_javascript/modules/components/img-popup.js

@@ -9,22 +9,28 @@ const lightImages = '.popup:not(.dark)';
 const darkImages = '.popup:not(.light)';
 let selector = lightImages;
 
-function updateImages(lightbox) {
+function updateImages(current, reverse) {
   if (selector === lightImages) {
     selector = darkImages;
   } else {
     selector = lightImages;
   }
 
-  lightbox.destroy();
-  lightbox = GLightbox({ selector: `${selector}` });
+  if (reverse === null) {
+    reverse = GLightbox({ selector: `${selector}` });
+  }
+
+  [current, reverse] = [reverse, current];
 }
 
 export function imgPopup() {
   if (document.querySelector('.popup') === null) {
     return;
   }
 
+  const hasDualImages = !(document.querySelector('.popup.light') === null &&
+    document.querySelector('.popup.dark') === null);
+
   if (
     (html.hasAttribute('data-mode') &&
       html.getAttribute('data-mode') === 'dark') ||
@@ -34,16 +40,18 @@ export function imgPopup() {
     selector = darkImages;
   }
 
-  let lightbox = GLightbox({ selector: `${selector}` });
+  let current = GLightbox({ selector: `${selector}` });
+  let reverse = null;
 
   if (document.getElementById('mode-toggle')) {
     window.addEventListener('message', (event) => {
       if (
         event.source === window &&
         event.data &&
-        event.data.direction === ModeToggle.ID
+        event.data.direction === ModeToggle.ID &&
+        hasDualImages
       ) {
-        updateImages(lightbox);
+        updateImages(current, reverse);
       }
     });
   }