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
* @see IPageChangedListener#pageChanged
     * 
     * @since 3.1
     */
    protected void firePageChanged(final PageChangedEvent event) {
        Object[] listeners = pageChangedListeners.getListeners();
        for (int i = 0; i < listeners.length; ++i) {
            final IPageChangedListener l = (IPageChangedListener) listeners[i];
            SafeRunnable.run(new SafeRunnable() {
                public void run() {
                    l.pageChanged(event);
                }
            });
        }
    }
	/**
	 * Adds a listener for page changes to the list of page changing listeners
	 * registered for this dialog. Has no effect if an identical listener is
	 * already registered.
	 * 
	 * @param listener
	 *            a page changing listener
	 */
	public void addPageChangingListener(IPageChangingListener listener) {
		pageChangingListeners.add(listener);		
	}
	/**
	 * Removes the provided page changing listener from the list of page
	 * changing listeners registered for the dialog.
	 *