Bug 142432 MasterPage from library can't be restored to modify
Details: MasterPage from library can't be restored to modify Step: 1.New a library named lib1, and insert a masterpage named lib-mp. 2.Publish the library into resource folder. 3.New a report design and use lib1. 4.Drop the lib-mp from library explorer into outline of report design. 5.Double click lib-mp, and switch to Property editro tab. 6.Click restore. Actual result: Pop up an error message and said that the master page cannot specify size the page type is not custom.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/DesignElementHandle.java
*         to a valid element.
	 */
	public DesignElementHandle getExtends( )
	{
		DesignElement parent = getElement( ).getExtendsElement( );
		if ( parent == null )
			return null;
		return parent.getHandle( parent.getRoot( ) );
	}
	/**
	 * Sets this element to extend the given element.
	 * 
	 * @param parent
	 *            handle to the element that this element is to extend. If
	 *            <code>null</code>, then this element will no longer extend
	 *            another element.
	 * @throws ExtendsException
	 *             if the parent element is of the wrong type as this element.
	 */
	public void setExtends( DesignElementHandle parent )
			throws ExtendsException
	{
		ExtendsCommand cmd = new ExtendsCommand( module, getElement( ) );
		cmd.setExtendsElement( parent );
	}
	/**
	 * Sets the name of the element that this element extends. The name must
	 * resolve to an element of the same type as this element.
	 * 
	 * @param name