How I solve this Null Pointer Exception in line 23?

Question

package application;

import java.net.URL;

import java.util.ResourceBundle;

import javafx.event.ActionEvent;

import javafx.fxml.FXML;

import javafx.scene.control.TextField;

import javafx.fxml.Initializable;

import model.Temperature;

public class TemperatureController implements Initializable {

@FXML private TextField t1;
@FXML private TextField t2;
@FXML private TextField t3;
private Temperature myTemperature;
@FXML
public void buttonCalculatePressed(ActionEvent event) {

    System.out.print("Convirtiendo...");
    double celsiusV = Double.parseDouble(t1.getText().toString());
    myTemperature.setCelsiusValue(celsiusV);
    double f = myTemperature.calculateFahrenheit();
    double k = myTemperature.calculateKelvin();
    t2.setText(f+"");
    t3.setText(k+"");
}

@Override
public void initialize(URL location, ResourceBundle resources) {
    myTemperature = new Temperature();
    assert t1 != null;
}

}