java.lang.NullPointerException when following CORBA architecture

Hello everybody!
I'm new to this forum, and I hope to pass a good time here!
the exercise demands to make a telephony program, in which two clients (owner, and answering machine), my problem is whenever one client tries to invoke any method that is not of type "void method(void)" it returns me a frightening exception:java.lang.NullPointerException
here is my code IDL

module globale{
struct msgAc{ //greeting message
string msgA;};
struct str{string name; string msg;};
typedef sequence <str> tabMsg;

struct tabMessage{
tabMsg tab;};
typedef sequence <string> redList;

interface clientutil{
exception {redListexp string m1;};
exception fullList{string m2;};
string getMsg();
boolean inListe(in string name);
void addMsg(in string s) raises (fullList,redListexp);};
interface client_proprietaire{
void emptyMsg();
void changeMsgAc(in string msg);
void addName(in string nom);
void getMsgByName(in string nom);
tabMessage getMsg();

};
};