You create a new ScoresController with a null parameter (first line in getScores()). What happens then, according to the stack trace you provided, is that this constructor calls another constructor in the same class, which tries to create a RequestController. I suspect that you are forwarding the null parameter to the RequestController constructor, which does not accept a null value for the observer parameter.
IllegalArgumentException usually denotes a bug, where a method receives a parameter it is not supposed to deal with.