Bug 38729 [Preferences] NPE PreferencePage isValid
getContainer may return null
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferencePage.java
* method returns the size set by <code>setSize</code>; if no size
	 * has been set, but the page has a UI control, the framework
	 * method <code>doComputeSize</code> is called to compute the size.
	 * </p>
	 *
	 * @return the size of the preference page encoded as
	 *   <code>new Point(width,height)</code>, or 
	 *   <code>(0,0)</code> if the page doesn't currently have any UI component
	 */
	public Point computeSize() {
		if (size != null)
			return size;
		Control control = getControl();
		if (control != null) {
			size = doComputeSize();
			return size;
		}
		return new Point(0, 0);
	}
	/**
	 * Contributes additional buttons to the given composite.
	 * <p>
	 * The default implementation of this framework hook method does
	 * nothing. Subclasses should override this method to contribute buttons 
	 * to this page's button bar. For each button a subclass contributes,
	 * it must also increase the parent's grid layout number of columns
	 * by one; that is,
	 * <pre>
	 * ((GridLayout) parent.getLayout()).numColumns++);
	 * </pre>