I thought I knew everything about Java but... it turns out you are allowed to have a public field with matching name and type in a class and in its superclass. And which one is being used depends on the type of the variable you use to access the field. That's why my mentions had wrong URLs in them.
Also this is pretty dumb. Generics work by generating casts at compile time. An empty list is type-agnostic because it's empty and throws an exception when you try adding anything to it. Yet Idea shows this warning, implying there's something fundamentally wrong with your code.