Bug 182190 [StatusHandling] [Wizards] getControl assert on wizard page has no message
M6 If you forget to set the control on your wizard page our assert gives no feedback. We need an error message.
bundles/org.eclipse.jface/src/org/eclipse/jface/wizard/WizardDialog.java
* @param listener
	 *            a page changing listener
	 */
	public void removePageChangingListener(IPageChangingListener listener) {
		pageChangingListeners.remove(listener);		
	}
	
	/**
	 * Notifies any page changing listeners that the currently selected dialog
	 * page is changing. Only listeners registered at the time this method is
	 * called are notified.
	 * 
	 * @param event
	 *            a selection changing event
	 * 
	 * @see IPageChangingListener#handlePageChanging(PageChangingEvent)
	 */
    protected void firePageChanging(final PageChangingEvent event) {
        Object[] listeners = pageChangingListeners.getListeners();
        for (int i = 0; i < listeners.length; ++i) {
            final IPageChangingListener l = (IPageChangingListener) listeners[i];
            SafeRunnable.run(new SafeRunnable() {
                public void run() {
               		l.handlePageChanging(event);
                }
            });
        }
    }
}