Strange NullPointerException

Hello,

I have taken one of the Swixml examples to write my own small Swing App. The 
GUI renders fine, but when implementing my first action I ran into a 
NullPointer exception... Here's my code:

public class ConfigBuilderUI extends WindowAdapter implements 
ActionListener{

 private SwingEngine ui;
 private JButton b_button;
 private JPanel p_config;

 public static void main(String[] args) throws Exception{
  new ConfigBuilderUI();
 }

 public AbstractAction submit = new AbstractAction() {
  public void actionPerformed(ActionEvent e) {
   //TODO: Some action takes place
  }
 };

 public ConfigBuilderUI() throws Exception{
  //Create ui
  ui = new SwingEngine(this);
  ui.render("xml/configbuilder.xml");

  //Init GUI components
  p_config.setToolTipText("JUPPIE");
  //Set visible
  ui.getRootComponent().setVisible(true);
 }

 /**
  * Invoked when an action occurs.
  */
 public void actionPerformed(ActionEvent e) {
  String command = e.getActionCommand();
  if ("AC_FILE_SELECT".equals( command )) {
   System.out.println("Button clicked!");
     }
 }
}

...when trying to "p_config.setToolTipText("JUPPIE");" the app throws the 
following exception:
java.lang.NullPointerException
at com.configbuilder.ui.ConfigBuilderUI.<init>(ConfigBuilderUI.java:50)
at com.configbuilder.ui.ConfigBuilderUI.main(ConfigBuilderUI.java:32)
Exception in thread "main"

I cannot understand this, because the documention says, that the gui 
components are initialized automatically.

I will appreciate any hints! Thanks in advance,

Sebastian