Update sidebar.js

fix bug https://github.com/docsifyjs/docsify/issues/1110

src/core/event/sidebar.js

@@ -38,7 +38,8 @@ export function collapse(el) {
   dom.on(el, 'click', ({ target }) => {
     if (
       target.nodeName === 'A' &&
-      target.nextSibling &&
+      target.nextSibling &&      
+      target.nextSibling.classList &&
       target.nextSibling.classList.contains('app-sub-sidebar')
     ) {
       dom.toggleClass(target.parentNode, 'collapse');