Q:

Unity Package Manager window not available in 2018.2.?

I have opened an existing project and the project shows an error. 
Here's the error:
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.

But I can't find any Package Manager window to fix this error and neither I'm sure there will be the built in package 'UI' in the Package Manager window.
What can I do to fix this error and compile the project?

A:

Deleting "Packages" directory helps if "Package manager" menu item is not there.

