Bug 38729 [Preferences] NPE PreferencePage isValid
getContainer may return null
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferencePage.java
* page-specific preference store. The default implementation
	 * 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 (IPreferencePageContainer) 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();