Bug 123219 Export data in attached file which has a table covers multiple page will throw exception.
Description: Export data in attached file which has a table covers multiple page will throw exception. Steps to reproduce: 1. Preview attached design file in viewer. 2. Select the table and click Export Data Expected result: No errors. Actual result: Exception Exception in: ReportEngineService.getMetaData( )
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/DataExtractionTask.java
return null;
	}
	
	/*
	 * check if the selected columns is valid, if no column is selected, then initialize the
	 * selected column using row expression.
	 */
	private void validateSelectedColumns( IBaseQueryDefinition query )
			throws EngineException
	{
		assert query != null;
		
		Collection exprs = getValueExpressions( query );
		if ( selectedColumns != null )
		{
			for ( int i = 0; i < selectedColumns.length; i++ )
			{
				boolean findColumn = false;
				Iterator iter = exprs.iterator( );
				while ( iter.hasNext( ) )
				{
					IScriptExpression expr = (IScriptExpression) iter.next( );
					if ( expr.getText( ).equalsIgnoreCase( selectedColumns[i] ) )
					{
						findColumn = true;
						break;
					}
				}
				if ( findColumn == false )
				{