How To Make Print Employee Button To Print Info Of Other Class

Basically the class is supposed to have a static variable that keeps track of the number of companies (numberOfCompanies) that have been created and a static method that returns the value of this variable. I need to have a constructor and a method to addEmployee, a method to printCompany, and a toString method. The addEmployee method will check that there is only 1 salesperson, 2 designers, and 4 manufacturing persons at a time and will check that there are no more than 7 employees of the company.

The addEmployee method will return a String indicating what the error is (i.e. "There is already a sales person in this company") or a null if the employee was added. If attempting to name a third company, an error message will be shown. what i have so far but my print company mmethod returns null company class :in which i am obliged to use inputdialog because it says that it cannot return void when i use the showmeassage diaolog.

company class:

import java.util.ArrayList;
import javax.swing.JOptionPane;
public class Company {
private ArrayList<Employees> list ;
private static int numberOfCompanies;

[Code] ....

error message when print button is clicked:

Exception in thread "AWT-EventQueue-0" java.lang.RuntimeException: JOptionPane: parentComponent does not have a valid parent
at javax.swing.JOptionPane.createInternalFrame(Unknown Source)
at javax.swing.JOptionPane.showInternalOptionDialog(Unknown Source)
at javax.swing.JOptionPane.showInternalMessageDialog(Unknown Source)

[Code] ......