Bug 136027 [Smoke]the report with a gounp can not preview
Description: the report with a gounp can not preview Steps to reproduce: 1. new a datasource and a dataset 2. drag the dataset to the layout 3. Add a group on the table. save and preview. Actual result: "Error happened while running the report" is appeared.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/GroupDialog.java
keyChooser = new Combo( composite, SWT.DROP_DOWN | SWT.READ_ONLY );
		keyChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		keyChooser.addModifyListener( new ModifyListener( ) {
			public void modifyText( ModifyEvent e )
			{
				if ( !keyChooser.getText( ).trim( ).equals( "" ) ) 
					resetInterval( );
			}
		} );
		
		Composite intervalArea = new Composite( composite, SWT.NONE );
		intervalArea.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		intervalArea.setLayout( UIUtil.createGridLayoutWithoutMargin( 2, false ) );
		Composite intervalTypeArea = new Composite( intervalArea, SWT.NONE );
		intervalTypeArea.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		intervalTypeArea.setLayout( UIUtil.createGridLayoutWithoutMargin( 1,
				false ) );
		Composite intervalRangeArea = new Composite( intervalArea, SWT.NONE );
		intervalRangeArea.setLayoutData( new GridData( GridData.HORIZONTAL_ALIGN_FILL ) );
		intervalRangeArea.setLayout( UIUtil.createGridLayoutWithoutMargin( 1,
				false ) );
		
		new Label( intervalTypeArea, SWT.NONE ).setText( GROUP_DLG_GROUP_INTERVAL_LABEL );
		new Label( intervalRangeArea, SWT.NONE ).setText( GROUP_DLG_GROUP_RANGE_LABEL );
		
		intervalType = new Combo( intervalTypeArea, SWT.READ_ONLY
				| SWT.DROP_DOWN );
		intervalType.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		for ( int i = 0; i < intervalChoices.length; i++ )
		{