[Java] Farenheit2Celcius konverterare, fixa ERROR vid inmatning av fel värde?

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class F2C extends JFrame{

	private String buttonTitleC2F = "C to F";
	private String buttonTitleF2C = "F to C";
	private JButton C2FButton = new JButton(buttonTitleC2F);
	private JButton F2CButton = new JButton(buttonTitleF2C);
	private JPanel panel = new JPanel();
	private JLabel label;
	private JTextField tempTextField;
	
		public F2C(){
			super ("Farenheit to Celcius converter");
			panel.setLayout(new FlowLayout()); 
			getContentPane().add(panel); 
			panel.setBackground(Color.gray);
			label = new JLabel(""); 
			tempTextField = new JTextField (5);
			
			panel.add(C2FButton);
			panel.add(F2CButton);
			panel.add(tempTextField);
			panel.add(label); 
			C2FButton.addActionListener(new ButtonListener());
			F2CButton.addActionListener(new ButtonListener2());
				
		}
		
		private class ButtonListener implements ActionListener {
			public void actionPerformed (ActionEvent evt){
				int tempFahr = (int) ((Double.parseDouble(tempTextField.getText())) * 1.8 +32);
				String tempF2C = String.valueOf(tempFahr);
				label.setText(tempF2C+ " C");
			}
		}
		
		private class ButtonListener2 implements ActionListener {
			public void actionPerformed (ActionEvent evt){
					int tempCelc = (int) (((Double.parseDouble(tempTextField.getText())) -32) /1.8);
					String tempC2F = String.valueOf(tempCelc);
					label.setText(tempC2F + " F");
			}
		}
		public static void main(String[] args) throws Exception{ 
			F2C panel = new F2C(); 
			panel.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 
			panel.setSize(400, 100); 
			panel.setVisible(true); 
		}
}