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
if( selectedColumns == null )
			{
				return null;
			}
			else
			{
				metaData = new ResultMetaData( null,
						selectedColumns );
				resultMetaList.add( metaData );
			}
		}
		return resultMetaList;
	}
	public void selectColumns( String[] columnNames )
	{
		selectedColumns = columnNames;
	}
	public IExtractionResults extract( ) throws EngineException
	{
		if( instanceId == null ) 
			return null;
		
		assert executionContext.getDataEngine() != null;
		DataEngine dataEngine = executionContext.getDataEngine().getDataEngine();
		
		ReportItemDesign rptItem = getDesignItem( instanceId.getComponentID() );
		assert rptItem != null;
		
		IBaseQueryDefinition query = rptItem.getQuery();
		validateSelectedColumns( query );