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?