This is correct:
Preferences prefs = Gdx.app.getPreferences("game.prefs");
But you are trying to do it before the library is initialized. At least thats what I can deduce being its just the line 11 in the Main java class.
Exception in thread "main" java.lang.NullPointerException at exampleGame.Main.main(Main.java:11)
All the Gdx.xxx will be null before the initialize is complete.