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
* @return
	 */
	private IChoice[] getSubIntervalChoice( )
	{
		Class columnType = getColumnType( );
		if ( columnType == null )
			return intervalChoicesAll;
		if ( String.class.isAssignableFrom( columnType ) )
		{
			return intervalChoicesString;
		}
		else if ( Date.class.isAssignableFrom( columnType ) )
		{
			return intervalChoicesDate;
		}
		else if ( Number.class.isAssignableFrom( columnType ) )
		{
			return intervalChoicesNumeric;
		}
		else
		{
			return intervalChoicesAll;
		}
	}
	private static IChoice[] getIntervalChoicesString( )
	{
		String[] str = new String[]{
				DesignChoiceConstants.INTERVAL_NONE,
				DesignChoiceConstants.INTERVAL_PREFIX,
		};