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
ResultMetaData metaData = null;
			if ( selectedColumns == null && instanceId != null )
			{
				try
				{
					ReportItemDesign rptItem = (ReportItemDesign) report.getReportItemByID( 
							instanceId.getComponentID( ) );
					assert rptItem != null;
					IBaseQueryDefinition query = rptItem.getQuery( );
					validateSelectedColumns( query );		
				}
				catch ( EngineException e )
				{
					e.printStackTrace( );
				}
			}
			
			if( selectedColumns == null )
			{
				return null;
			}
			else
			{
				metaData = new ResultMetaData( null,
						selectedColumns );
				resultMetaList.add( metaData );
			}
		}
		return resultMetaList;
	}