check for sidebar existence, fixes #2918

resources/assets/javascripts/lib/sidebar.js

@@ -2,8 +2,11 @@ const Sidebar = {
 
     place() {
         const header = document.getElementById('main-header');
-        document.getElementById('sidebar').style.top =
+        const sidebar = document.getElementById('sidebar');
+        if (sidebar) {
+            sidebar.style.top =
                 header.offsetTop + header.offsetHeight + 'px';
+        }
     },
 
     observeSidebar() {
@@ -16,8 +19,11 @@ const Sidebar = {
         /**
          * Observe if sidebar fits into viewport.
          */
+        const sidebar = document.getElementById('sidebar');
+        if (sidebar) {
             const sObserver = new IntersectionObserver(STUDIP.Sidebar.fits, options);
-        sObserver.observe(document.getElementById('sidebar'));
+            sObserver.observe();
+        }
     },
 
     observeBody() {
@@ -87,6 +93,7 @@ const Sidebar = {
 
     footerVisible(entries, observer) {
         const sidebar = document.getElementById('sidebar');
+        if (sidebar) {
             entries.forEach(entry => {
                 // Footer is visible on current page.
                 if (entry.isIntersecting) {
@@ -100,6 +107,7 @@ const Sidebar = {
                 }
             });
         }
+    }
 };
 
 export default Sidebar;