nullpointexception while changing the text of fxml label in controller

@FXML public ComboBox<Integer> baudRateCombo;
@FXML public ComboBox<String> serialPortCombo;
@FXML public Spinner propotional;
@FXML public Spinner integral;
@FXML public Spinner derivative;
@FXML public Label lastCmdStatus;
@FXML public Label tempReading;
@FXML public Label errorReading;
private final Logger logger = LoggerFactory.getLogger(getClass());
private ArduinoInterfaceControl arduinoInterfaceControl;
private Stage stage;
private String prop;
private String inte;
private String deri;
private boolean lastCommandSuccessful;
public static final String LOCAL_SIMULATOR = "Local Simulator";
private String[] params;
private ArduinoInterfaceControl arduinoInterface;
public SerialCommunicationController() {
}
public SerialCommunicationController(String cmd){
    setValues(cmd);
}
private Stage dialogStage;
public void setDialogStage(Stage dialogStage) {
    this.dialogStage = dialogStage;
}

public void initialise() {
    baudRateCombo.getItems().add(9600);
    baudRateCombo.getItems().add(19200);
    Collection<String> availablePorts = NRSerialPort.getAvailableSerialPorts();
    serialPortCombo.getItems().addAll(availablePorts);

}

public void connectPressed(ActionEvent actionEvent) {
    System.out.print("connecting");
    if(!settingCombinationsAreAllowed()) {
        showSerialSettingErrorDialog();
        return;
    }

    arduinoInterface = new RS232Controller(serialPortCombo.getValue(), baudRateCombo.getValue());
}

private void showSerialSettingErrorDialog() {
    Alert alert = new Alert(Alert.AlertType.INFORMATION);
    alert.setTitle("Invalid serial settings");
    alert.setHeaderText("Invalid serial settings entered");
    alert.setContentText("Please ensure you have entered a port and speed.");
    alert.showAndWait();
}

private boolean settingCombinationsAreAllowed() {
    return baudRateCombo.getValue() != null && serialPortCombo.getValue() != null;
}

public void startReading(ActionEvent actionEvent) {
}

public void stopReading(ActionEvent actionEvent) {
}

public void plotGraph(ActionEvent actionEvent) {
}

public void setPointTemp(ActionEvent actionEvent) {
}

public void setPIDVal(ActionEvent actionEvent) {
    System.out.print(propotional.getValue());
    prop = propotional.getValue().toString();
    inte = integral.getValue().toString();
    deri = derivative.getValue().toString();
    String text = ("p"+prop+"p"+inte +"i"+ deri+"d");
    if(Strings.isNullOrEmpty(text) || text.contains("\n")) {
        logger.info("Pressed send message with no text.");

        Alert alert = new Alert(Alert.AlertType.INFORMATION);
        alert.setTitle("Message text error");
        alert.setHeaderText("Invalid message entered");
        alert.setContentText("Please ensure you have entered some text, and it does not contain line feeds.");
        alert.showAndWait();
    }
    else {
        arduinoInterface.sendPrintCommand(text);
        enterWaitingState();
    }
}

public ArduinoInterfaceControl getArdiunoInterface() {
    return arduinoInterface;
}
private void enterWaitingState() {
    lastCmdStatus.setText("Waiting..");
}
@FXML
public void setValues(String readings){
    String[] params = readings.split("\\s+");
    // something went wrong, just decode to blank.
    if(params.length < 1) {
        lastCommandSuccessful = false;
    }
    else {
        tempReading.setText(params[0]);
        errorReading.setText(params[1]);
        // successful command received.
        lastCommandSuccessful = Boolean.valueOf(params[1]);
    }
}