Bug 41927 [Dialogs] Selecting in PreferenceDialog generates too many invalid entries
20030825 If you select an entry, close the dialog and then reopen it selecting anotehr entry will result in a "this page has invalid values" although you selected nothing.
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferencePage.java
* returns <code>null</code>.
	 * </p>
	 *
	 * @return the preference store, or <code>null</code> if none
	 */
	protected IPreferenceStore doGetPreferenceStore() {
		return null;
	}
	/**
	 * Returns the container of this page.
	 *
	 * @return the preference page container, or <code>null</code> if this
	 *   page has yet to be added to a container
	 */
	public IPreferencePageContainer getContainer() {
		return container;
	}
	/**
	 * Returns the preference store of this preference page.
	 *
	 * @return the preference store , or <code>null</code> if none
	 */
	public IPreferenceStore getPreferenceStore() {
		if (preferenceStore == null)
			preferenceStore = doGetPreferenceStore();
		if (preferenceStore != null)
			return preferenceStore;
		else if (container != null)
			return container.getPreferenceStore();
		return null;