1. Use singleton design pattern(have a look to
http://www.patterndepot.com/put/8/Singleton.PDF) for creating if you want to
create only one instance of a class.
If it is not easy to understand you can do sometihng like that
Use a handle in the class which listens the action of your button but it should
be static (such as static boolean isWindowCreated)
Set this flag when you create a window to true.And control the state of this
flag before handling the event of your button.An example  code is in below:
public class JFrmMainWindow extends JFrame implements

static boolean isWindowCreated=false;
java.awt.event.ActionListener{
JButton newWindow = new JButton();
public JFrmMainWindow() {
init();
}
public static void main(String[] args) {
JFrmMainWindow jFrmMainWindow = new JFrmMainWindow();

}
private void init() {
newWindow.setBounds(new Rectangle(134, 211, 132, 45));
newWindow.setText("New Window");
newWindow.addActionListener(this);
this.setSize(500,400);
this.getContentPane().setLayout(null);
this.getContentPane().add(newWindow, null);
this.setVisible(true);
}

private void createNewWindowAndShowIt(){
if(!isWindowCreated){
JFrame newWindow=new JFrame();
newWindow.setSize(100,100);
newWindow.setVisible(true);
isWindowCreated=true;
}
}
public void actionPerformed(ActionEvent e) {
createNewWindowAndShowIt();
}
}

2. Null pointer exception means you did not create an instance but you are
trying to use it.
E.g
SimpleClass anInstanceOfSimpleClass;
anInstanceOfSimpleClass.makeASimpleThing();

3.You can navigate a folder with javax.swing.JFileChooser
Maybe you can navigate a web page with Runtime.getRuntime().exec method you can
execute iexplore but you can't naigate a web a page with swing  directly.maybe
there can be some libs maybe you can look to www.sf.net

Your questions are not clear maybe you can put some example codes to your mails