Unity UI How do I access UnityEngine.UI in script in Unity 2019.2?

I knew that in Unity 2019.2, the UnityEngine.UI was replaced by UnityEngine.UIElements and it was moved to the Packages, which resulted in that I could not access it in script anymore. However, one of the plugins I am using is a Soft Mask plugin that extends the Mask class in UnityEngine.UI. It is now completely broken. I tried to fix it with UIElements, but I didn't find anything that is related to Mask. Therefore, I want to ask how I can access the UnityEngine.UI from the Packages? It's inside the Packages so maybe there is a way to access it in script, right?

 

I think I found the reason. It's because the com.unity.ugui folder inside the Packages folder in the Unity project folder does not have any scripts, which the Visual Studio cannot find them so it cannot get the references. I don't know why this happened though...