Bug 121687 Exception throws when export data in table which inside a grid cell.
Description: Exception throws when export data in table which inside a grid cell. Steps to reproduce: 1. insert a grid. 2. insert one table to one grid cell. 3. insert another table to the other grid cell. 4. export data in two tables. Expected result: Data are exported. Actual result: Exception in: ReportEngineService.getMetaData( )
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/parser/EngineIRVisitor.java
* create a list band using the items in slot.
	 * 
	 * @param elements
	 *            items in DE's IR
	 * @return ListBand.
	 */
	private ListBandDesign createListBand( SlotHandle elements )
	{
		ListBandDesign band = new ListBandDesign( );
		for ( int i = 0; i < elements.getCount( ); i++ )
		{
			apply( elements.get( i ) );
			if ( currentElement != null )
			{
				band.addContent( (ReportItemDesign) currentElement );
			}
		}
		return band;
	}
	/**
	 * create a list group using the DE's ListGroup.
	 * 
	 * @param handle
	 *            De's list group
	 * @return engine's list group
	 */
	public void visitListGroup( ListGroupHandle handle )
	{
		ListGroupDesign listGroup = new ListGroupDesign( );
		setupGroup( listGroup, handle );