UnityEditor.UI.dll' is in timestamps but is not known in assetdatabase

网上找到一篇文章：

一、Remove the UI System
1、Close Unity
2、Access on Unity's installation folder on Explorer or Finder on mac (On mac rightclick on the App and click on ShowPackageContent).
3、Locate the folder named "UnityExtensions/Unity". (On Mac: "Content/UnityExtensions/Unity".
4、In this folder you will notice a folder named "GUISystem"
5、Move this folder from the installation folder (you may need administrator rights). Yes, right. Do not just rename it or move it out the "Unity" folder, Unity will find it... I assure you...
二、Have Unity notice the change
1、Open Unity again in the project you had the issue
2、You may have an error "GameObject (named 'Canvas') references runtime script in scene file. Fixing!"
3、Close Unity, DO NO SAVE ANY SCENE
三、Restore the UI System
1、Make sure Unity is closed
2、Restore the "GUISystem" to its previous location
3、Open Unity again
4、Unity will compile script
5、You will see the warning "Timestamps (19) and assets (21) maps out of sync." and the errors "Asset '' is in assets but has no assettimestamp..." just ignore them
6、Everything should be ok again

翻译一下就是：

1、关闭Unity

先找到UnityExtensions/Unity下的GUISystem目录，删除它，一定要删除，不要移动到别的文件夹

2、打开Unity

出现"GameObject(named ‘Canvas‘)的错误信息，修复它，关闭Unity

3、再从回收站将GUISystem恢复到原来目录

从新打开Unity即可

UnityEditor.UI.dll' is in timestamps but is not known in assetdatabase