fix tabs_bar.js (#4436)

app/javascript/mastodon/features/ui/components/tabs_bar.js

@@ -48,7 +48,9 @@ export default class TabsBar extends React.Component {
       const nextTab = tabs.find(tab => tab.contains(e.target));
       const { props: { to } } = links[Array(...this.node.childNodes).indexOf(nextTab)];
 
-      currentTab.classList.remove('active');
+      if (currentTab) {
+        currentTab.classList.remove('active');
+      }
 
       const listener = debounce(() => {
         nextTab.removeEventListener('transitionend', listener);