String JE = tFJmlEmas.getString();
                String JP = tFJmlPerak.getString();
                String HE = tFHrgEmas.getString();
                String HP = tFHrgPerak.getString();
                String NE = tFNisabEmas.getString();
                String NP = tFNisabPerak.getString();


                double ZEmas = 0, ZPerak = 0, uangEmas = 0, uangPerak = 0;
                try {

                    if (JE.equals("") || JP.equals("")|| HE.equals("")|| HP.equals("")|| NP.equals("")|| HP.equals("")) {
                        alert = new Alert("Error", "You should enter Username and Location", null, AlertType.ERROR);
                        alert.setTimeout(Alert.FOREVER);
                        myDisplay.setCurrent(alert);
                    } else {
                        double jmlEmas = Double.parseDouble(JE);
                        double jmlPerak = Double.parseDouble(JP);
                        double hrgEmas = Double.parseDouble(HE);
                        double hrgPerak = Double.parseDouble(HP);
                        double NisabEmas = Double.parseDouble(NE);
                        double NisabPerak = Double.parseDouble(NP);

                        if (jmlPerak >= NisabPerak) {
                            ZPerak = 0.025 * jmlPerak;
                            uangPerak = 0.025 * jmlPerak * hrgPerak;
                            tFZPerak.setString(String.valueOf(ZPerak) + " gram atau " + String.valueOf(uangPerak) + " Rupiah");

                        } else {
                            tFZPerak.setString("Anda Tidak Wajib Zakat");
                        }
                        if (jmlEmas >= NisabEmas) {
                            ZEmas = 0.025 * jmlEmas;
                            uangEmas = 0.025 * jmlEmas * hrgEmas;
                            tFZEmas.setString(String.valueOf(ZEmas) + " gram atau " + String.valueOf(uangEmas) + " Rupiah");
                        } else {
                            tFZEmas.setString("Anda Tidak Wajib Zakat");

                        }

                    }


                } catch (NumberFormatException e) {
                }


terus error e :

TRACE: <at java.lang.NullPointerException:   0>, Exception caught in Display class
java.lang.NullPointerException:   0