Blackberry Field Manager Scrolling Shows NullpointerException - blackberry

Question

I am having problem with Blackberry horizontal and vertical Field Managers. In my sample application I have two vertical field managers added to one horizontal field manager and the GUI is in the following figure.
In the field change listener of button I am removing the left VFM, then only the Home button is displayed, and in home button listener I am adding the left VFM, then it get back to the above screen.
The problem is occurring after scrolling the left content and deleting the left VFM, Then only Home button is displayed and track pad navigation (scrolling) leads to Null Pointer exception screen. The following figure contains the simulator screen shot of error.
And here is the complete source code for reproducing the error.
public class MyApp extends UiApplication{
public static void main(String[] args)
{
// Create a new instance of the application and make the currently
// running thread the application's event dispatch thread.
MyApp theApp = new MyApp();
theApp.enterEventDispatcher();
}
/**
* Creates a new MyApp object
*/
public MyApp()
{
// Push a screen onto the UI stack for rendering.
pushScreen(new MyScreen());
}
}
public final class MyScreen extends MainScreen{
private HorizontalFieldManager hfmMainManager = null;
private VerticalFieldManager menuButtonManager = null;
private String[] buttons = new String[]{"BUTTON ID 1",
"BUTTON ID 2",
"BUTTON ID 3",
"BUTTON ID 4",
"BUTTON ID 5",
"BUTTON ID 6",
"BUTTON ID 7",
"BUTTON ID 8",
"BUTTON ID 9",
"BUTTON ID 10"};
private boolean menuShowing = false;
/**
* Creates a new MyScreen object
*/
public MyScreen()
{
super( Manager.NO_VERTICAL_SCROLL | MainScreen.NO_VERTICAL_SCROLLBAR
| Manager.NO_HORIZONTAL_SCROLL | Manager.NO_HORIZONTAL_SCROLLBAR);
hfmMainManager = new HorizontalFieldManager(Manager.NO_HORIZONTAL_SCROLL
| Manager.NO_HORIZONTAL_SCROLLBAR);
////////////////////MENU BUTTONS
menuButtonManager = new VerticalFieldManager(Manager.NO_VERTICAL_SCROLL | Manager.NO_VERTICAL_SCROLLBAR);
menuButtonManager.add(new ButtonField("Header Button", Field.FIELD_HCENTER));
VerticalFieldManager vfmScrollable = new VerticalFieldManager(Manager.VERTICAL_SCROLL | Manager.VERTICAL_SCROLLBAR);
for (int i = 0; i < buttons.length; i++) {
ButtonField buttonField = new ButtonField(buttons[i], Field.FIELD_HCENTER);
buttonField.setChangeListener(listenerButton);
vfmScrollable.add(buttonField);
}
menuButtonManager.add(vfmScrollable);
hfmMainManager.add(menuButtonManager);
VerticalFieldManager vfmMenuContentManager = new VerticalFieldManager(Manager.NO_VERTICAL_SCROLLBAR | Manager.NO_VERTICAL_SCROLL);
ButtonField buttonField = new ButtonField("Home Button", Field.FIELD_HCENTER);
buttonField.setChangeListener(listenerButton);
buttonField.setMargin(10, 0, 10, 0);
vfmMenuContentManager.add(buttonField);
hfmMainManager.add(vfmMenuContentManager);
add(hfmMainManager);
menuShowing = true;
}
private FieldChangeListener listenerButton = new FieldChangeListener() {
public void fieldChanged(Field arg0, int arg1) {
synchronized (UiApplication.getEventLock()) {
if(menuShowing){
menuShowing = false;
hfmMainManager.delete(menuButtonManager);
}else{
menuShowing = true;
hfmMainManager.insert(menuButtonManager, 0);
}
}
}
};
}
How do I solve this Issue?
Thanks in advance
Regards
Anish

Answer 1

I've seen similar behavior when trying to change the field structure from a focus or unfocus event. When a field removes itself from the screen in the middle of the focus event, the focus tracking can get out of sync, and cause unusual errors like this. Try changing from grabbing the UI lock to calling invokeLater instead. That way the change listener can finish executing before you delete the VFM, keeping all the invariants in place.
Get rid of this:
synchronized (UiApplication.getEventLock()) {
Replace it with this:
UiApplication.getInstance().invokeLater(new Runnable{ public void run() {