Bug 121682 Exception throws when export data from table containing highlight/map.
Description: Exception throws when export data from attached report. Steps to reproduce: 1. I add a table which has a group, a highlight rule and a map rule in group row. 2. Preview the report attached. 3. Export the data in table. 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
public DataExtractionTask( ReportEngine engine, IReportRunnable runnable,
			ReportDocumentReader reader )
	{
		super( engine, runnable );
		
		
		this.reportDocReader = reader;
		executionContext.setReportDocument( reportDocReader );
		executionContext.setFactoryMode( false );
		executionContext.setPresentationMode( true );
		report = new ReportParser( ).parse( ( (ReportRunnable) runnable )
				.getReport( ) );
		
		executionContext.setReport( report );
		setParameterValues( reportDocReader.getParameterValues( ) );
		IDataEngine dataEngine = executionContext.getDataEngine();
		dataEngine.prepare( report, context );
	}
	
	public void setInstanceID( InstanceID iid )
	{
		instanceId = iid;
	}
	public List getMetaData( )
	{
		if ( resultMetaList == null )
		{
			resultMetaList = new ArrayList( );