I get a NullPointerException calling a Superclass Method in Subclass Inner Class Constructor... What's the Deal?

In my application's main class (subclass of Application), I have a public inner class that simply contains 3 public string objects. In the parent class I declare an object of that inner class.

public class MainApplication extends Application {
    public class Data {
        public String x;
        public String y;
        public String z;
    }

    private Data data;

    MainApplication() {
        data = new Data()

        data.x = SuperClassMethod();
    }
}

After I instantiate the object in the constructor, I get a runtime error when I try to assign a value in the inner class with a superclass method.

Any idea what's up here?? Can you not call superclass methods in the subclass constructor?

** Edit ** Original question was about inner class member assignment in outer class constructor. Turned out the issue was with calling a superclass method in the class's constructor. It was giving me a null pointer exception. Thus, the question has changed.