[BUG] Output node not working

Output node did not preview the group output

1. Create a node group
2. After saving and modify or add a new output to the output node
3. It is not displaying the output while set preview

*It was working fine until I added a new output node to the output group node and the node until the set preview before the output node works fine


**Desktop (please complete the following information):**
- OS: Windows 10
- Blender Version 2.83.1
- Sorcar Version 3.2

FIX I USE
* I used to copy and repaste the whole tree in another group
* Can someone tell how to add the output node back if deleted

该 issue 涉及 node group 中的 Output node 在修改后不显示预览的问题。查看 Sorcar addon 代码中对 node group 的处理以及 output node 的预览逻辑。首先检查 node group 创建函数和 output node 更新函数。测试用户描述的修复方法（复制并重新粘贴 tree），以了解底层问题。

由索引模型根据 Issue 内容生成。