Bug 38729 [Preferences] NPE PreferencePage isValid
getContainer may return null
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferencePage.java
/**
	 * Whether this page has the standard Apply and Defaults buttons; 
	 * <code>true</code> by default.
	 *
	 * @see #noDefaultAndApplyButton
	 */
	private boolean createDefaultAndApplyButton = true;
	/**
	 * Standard Defaults button, or <code>null</code> if none.
	 * This button has id <code>DEFAULTS_ID</code>.
	 */
	private Button defaultsButton = null;
	/**
	 * The container this preference page belongs to; <code>null</code>
	 * if none.
	 */
	private IPreferencePageContainer container = null;
	/**
	 * Standard Apply button, or <code>null</code> if none.
	 * This button has id <code>APPLY_ID</code>.
	 */
	private Button applyButton = null;
	/**
	 * Description label.
	 * 
	 * @see #createDescriptionLabel.
	 */
	private Label descriptionLabel;
	/**
	 * Caches size of page.