fix[Sidebar]: data should return a object

Author: PanJiaChen

src/views/layout/components/Sidebar/SidebarItem.vue

@@ -67,6 +67,7 @@ export default {
     // To fix https://github.com/PanJiaChen/vue-admin-template/issues/237
     // TODO: refactor with render function
     this.onlyOneChild = null
+    return {}
   },
   methods: {
     hasOneShowingChild(children, parent) {