Bug 124251 the data cann't been exported.
Description: the data cann't been exported. Steps to reproduce: 1. new a report 2. new a datasource and a data set(DS_A) 3. use an external library 4. drag a table to the report layout from the imported library 5. insert an another table into the layout of the report and bind the data set(DS_A) 6. preview in the 'Web Viewer' and export the data Actual result: The data can been exported. Expect result: The data can been exporteded
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/DataExtractionTask.java
resultSetName = (String)mapDispNameToResultSetName.get( displayName );
		selectedColumns = null;
		currentResult = null;
	}
	
	public List getMetaData( ) throws EngineException
	{
		if ( resultMetaList == null )
		{
			resultMetaList = new ArrayList( );
			ResultMetaData metaData = null;
			if ( selectedColumns == null && instanceId != null )
			{
				currentResult = extract( );
				try
				{
					resultMetaList.add( currentResult.getResultMetaData( ) );
				}
				catch( BirtException be )
				{
					be.printStackTrace();
				}
			}
			
			if( selectedColumns == null )
			{
				return null;
			}
			else
			{