Bug 135465 [smoke]An error occurred when new a casading Parameter
Description: An error occurred when new a casading Parameter Step to reproduce: 1.new a sample db data source and data set 2.New a cascading parameter group, named "gp1", 3.select the data set, then create two cascading parameters, one is row["customernumber"], named p1; the other is row["customername"] Actual result; error message occurred repeatly
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/CascadingParametersDialog.java
String displayFormat;
		IChoiceSet choiceSet = getFormatChoiceSet( getSelectedDataType( ) );
		if ( choiceSet == null )
		{
			displayFormat = DesignEngine.getMetaDataDictionary( )
					.getChoiceSet( DesignChoiceConstants.CHOICE_STRING_FORMAT_TYPE )
					.findChoice( DesignChoiceConstants.STRING_FORMAT_TYPE_UNFORMATTED )
					.getDisplayName( );
		}
		else
		{
			displayFormat = choiceSet.findChoice( formatCategroy )
					.getDisplayName( );
			if ( formatPattern != null )
			{
				displayFormat += ":  " + formatPattern; 
			}
		}
		formatField.setText( "" + displayFormat ); 
		changeFormat.setEnabled( choiceSet != null );
		if ( selectedParameter != null )
		{
			try
			{
				doPreview( formatPattern == null ? formatCategroy
						: formatPattern );
				String format = formatCategroy;
				if ( formatPattern != null )
				{
					format += ":" + formatPattern;