'Main.this' cannot be referenced from a static context if outer class is generified

Why is the following code ok, but as soon as T is added to Main as a generic, it throws the following error?

'Main.this' cannot be referenced from a static context

//public class Main<T> { - uncomment this for the error to appear

public class Main {
    public static void main(String[] args) {
        new Main();
    }

    class TestNonStatic {}

    private static class TestStatic {
        public TestStatic(TestNonStatic nonStatic) { //this is the line that fails

        }
    }
}

I went through generics restrictions but I don't get it why this results into an error. I also went through a lot of similar questions but I don't understand why adding the generic would change the situation.