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
IDataEngine dataEngine = executionContext.getDataEngine();
		dataEngine.prepare( report, context );
	}
	
	public void setInstanceID( InstanceID iid )
	{
		instanceId = iid;
	}
	public List getMetaData( )
	{
		if ( resultMetaList == null )
		{
			resultMetaList = new ArrayList( );
			ResultMetaData metaData = null;
			if ( selectedColumns == null && instanceId != null )
			{
				try
				{
					ReportItemDesign rptItem = getDesignItem( instanceId
							.getComponentID( ) );
					assert rptItem != null;
					IBaseQueryDefinition query = rptItem.getQuery( );
					validateSelectedColumns( query );		
				}
				catch ( EngineException e )
				{
					e.printStackTrace( );
				}
			}