Bug 38729 [Preferences] NPE PreferencePage isValid
getContainer may return null
bundles/org.eclipse.jface/src/org/eclipse/jface/preference/PreferencePage.java
* This is a framework hook method for sublcasses to do special things when
	 * the Defaults button has been pressed.
	 * Subclasses may override, but should call <code>super.performDefaults</code>.
	 * </p>
	 */
	protected void performDefaults() {
		updateApplyButton();
	}
	/** 
	 * Method declared on IPreferencePage.
	 * Subclasses should override
	 */
	public boolean performOk() {
		return true;
	}
	/** (non-Javadoc)
	 * Method declared on IPreferencePage.
	 */
	public void setContainer(IPreferencePageContainer container) {
		this.container = container;
	}
	/**
	 * The <code>PreferencePage</code> implementation of this method 
	 * declared on <code>DialogPage</code> updates the container.
	 */
	public void setErrorMessage(String newMessage) {
		super.setErrorMessage(newMessage);
		if (getContainer() != null) {
			getContainer().updateMessage();
		}