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/api/impl/DataExtractionTask.java
}
	
	private void populateContent( )
	{
		if( contentMap == null )
			contentMap = new HashMap( );
		
		for( int i=0; i<report.getContentCount(); i++)
		{
			ReportItemDesign rptItem = report.getContent( i );
			contentMap.put(new Long( rptItem.getID()), rptItem);
		}
	}
	
	private ReportItemDesign getDesignItem( long componentId )
	{
		assert contentMap != null;
		return (ReportItemDesign)contentMap.get(new Long( componentId ));
	}
	
	private void validateSelectedColumns( IBaseQueryDefinition query )
			throws EngineException
	{
		assert query != null;
		Collection exprs = query.getRowExpressions( );
		if ( selectedColumns != null )
		{
			for ( int i = 0; i < selectedColumns.length; i++ )
			{
				boolean findColumn = false;