[skip ci] Fix sidebar on assignees page

app/assets/javascripts/sidebar/sidebar_bundle.js

@@ -8,7 +8,14 @@ document.addEventListener('DOMContentLoaded', () => {
   const mediator = new Mediator(gl.sidebarOptions);
   mediator.fetch();
 
-  new Vue(sidebarAssignees).$mount('#js-vue-sidebar-assignees');
+  const sidebarAssigneesEl = document.querySelector('#js-vue-sidebar-assignees');
+
+  // Only create the sidebarAssignees vue app if it is found in the DOM
+  // We currently do not use sidebarAssignees for the MR page
+  if (sidebarAssigneesEl) {
+    new Vue(sidebarAssignees).$mount(sidebarAssigneesEl);
+  }
+
   new Vue(sidebarTimeTracking).$mount('#issuable-time-tracker');
 });
 

app/assets/javascripts/sidebar/stores/sidebar_store.js

@@ -19,10 +19,12 @@ export default class SidebarStore {
   }
 
   processUserData(data) {
-    this.renderedUsers = data.assignees;
+    if (data.assignees) {
+      this.renderedUsers = data.assignees;
 
-    this.removeAllUserIds();
-    this.renderedUsers.map(u => this.addUserId(u.id));
+      this.removeAllUserIds();
+      this.renderedUsers.map(u => this.addUserId(u.id));
+    }
   }
 
   processTimeTrackingData(data) {