fix tab behaviour

src/mixins/navigation.js

@@ -13,12 +13,12 @@ export default {
   },
   methods: {
     toggleSheet(itemIndex) {
-      if (this.tab !== 'contents') {
-        const link = this.itemurls[itemIndex];
-        window.location.hash = `selectedItem-${link}`;
+      const link = this.itemurls[itemIndex];
+      const tree = document.getElementsByClassName('view-tree');
 
-        const tree = document.getElementsByClassName('view-tree')[0];
-        tree.scrollBy(0, -80);
+      if (tree.length) {
+        window.location.hash = `selectedItem-${link}`;
+        tree[0].scrollBy(0, -80);
       }
 
       this.sequenceindex = this.computedsequenceindex;