Error running compositeDisposable in an Android fragment

I hope you can help me :) I've been stuck in this and some time. I am running an app that fills Mysql with data and when I try the method in an Activity it works normally, but when I execute it in a fragment it gives me an error when I execute it.

This is the code of my fragment.

public class Selection3 extends Fragment {
ImageView icon_card, icon_efectivo;
TextView efectivo, card;
Button botonlisto;
LinearLayout seleccioncard;
CompositeDisposable compositeDisposable;
KAPI mService;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    View v = inflater.inflate(R.layout.fragment_selection3, container, false);
    icon_card=(ImageView) v.findViewById(R.id.icon_card);
    seleccioncard=(LinearLayout) v.findViewById(R.id.seleccion_card);
    seleccioncard.setVisibility(View.INVISIBLE);
    icon_efectivo=(ImageView) v.findViewById(R.id.icon_efectivo);
    efectivo=(TextView) v.findViewById(R.id.txt_efectivo);
    card=(TextView) v.findViewById(R.id.txt_card);
    botonlisto=(Button) v.findViewById(R.id.botonlisto);
    botonlisto.setClickable(true);

    //BOTON RESUMIR COMPRA
    botonlisto.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            placeOrder();
            initDB();


        }
    });
    return v;
}
private void initDB() {
    Common.edmtRoomDatabase = EDMTRoomDatabase.getInstance(getActivity());
    Common.cartRepository = CartRepository.getInstance(CartDataSource.getInstance(Common.edmtRoomDatabase.cartDAO()));
    Common.favoriteRepository = FavoriteRepository.getInstance(FavoriteDataSource.getInstance(Common.edmtRoomDatabase.favoriteDAO()));
}

private void placeOrder() {
    //DIALOGO PARA CONFIRMAR  COMPRA Y SUBIR A MYSQL
    AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
    builder.setTitle("Confirmar Orden");
    View submit_order_layout = LayoutInflater.from(getActivity()).inflate(R.layout.submit_order,null);
    final EditText edt_comment = (EditText)submit_order_layout.findViewById(R.id.edt_comment);
    builder.setView(submit_order_layout);

    builder.setNegativeButton("Cancelar", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            dialog.dismiss();
        }
    }).setPositiveButton("OK", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            final String orderComment = edt_comment.getText().toString();
            final String orderPhone =edt_comment.getText().toString();//telefono no asignado /prueba con comments;

            //Submit order AQUI ME MUESTRA EL ERROR (linea134)
            compositeDisposable.add(
                    Common.cartRepository.getCartItems()
                            .observeOn(AndroidSchedulers.mainThread())
                            .subscribeOn(Schedulers.io())
                            .subscribe(new Consumer<List<Cart>>() {
                                @Override
                                public void accept(List<Cart> carts) throws Exception {
                                    if (!TextUtils.isEmpty(orderPhone))
                                        sendOrderToServer(Common.cartRepository.sumPrice(),
                                                carts,
                                                orderComment,orderPhone);
                                    else
                                        Toast.makeText(getActivity(),"Ingrese un contacto",Toast.LENGTH_SHORT).show();                                    }
                            })
            );
        }
    });
    builder.show();

}

private void sendOrderToServer(float sumPrice, List<Cart> carts, String orderComment, String orderPhone) {
    if (carts.size()>0)
    {
        String orderDetail = new Gson().toJson(carts);
        //llenado detalles de la compra con phone por prueba
        mService.submitOrder(String.valueOf(sumPrice),orderDetail,orderComment,orderPhone,orderPhone)
                .enqueue(new Callback<String>() {
                    @Override
                    public void onResponse(Call<String> call, Response<String> response) {
                        Toast.makeText(getActivity(),"Orden Ingresada",Toast.LENGTH_SHORT).show();
                        //clear Cart
                        Common.cartRepository.emptyCart();
                    }

                    @Override
                    public void onFailure(Call<String> call, Throwable t) {
                        Log.e("ERROR",t.getMessage());

                    }
                });

    }

}

and this is the error that shows me

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean io.reactivex.disposables.CompositeDisposable.add (io.reactivex.disposables.Disposable)' on a null object reference         at com.xxxxx.fragment.Selection3 $ 4.onClick (Selection3.java:134)

Thank you very much for your help