fix[Sidebar]: fixed infinite loop bug(#1333)

* fixed infinite loop Bug when in hasOneShowingChild Edit the onlyOneChild

* tweak

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

@@ -64,9 +64,9 @@ export default {
     }
   },
   data() {
-    return {
-      onlyOneChild: null
-    }
+    // To fix https://github.com/PanJiaChen/vue-admin-template/issues/237
+    // TODO: refactor with render function
+    this.onlyOneChild = null
   },
   methods: {
     hasOneShowingChild(children, parent) {