我打开了一个现有项目,该项目显示错误。这是错误:
Assets/unity-ui-extensions/Scripts/Effects/RaycastMask.cs(34,47): error CS1070: The type `UnityEngine.ICanvasRaycastFilter' has been forwarded to an assembly that is not referenced. Enable the built in package 'UI' in the Package Manager window to fix this error.
但我找不到任何包管理器窗口来修复此错误,我也不确定包管理器窗口中是否会有内置包“UI”。
我该怎么做才能修复此错误并编译项目?

