Bug 146210 [Regression]"null" value will appear in parameter "display as" box several times
Description:"null" value will appear in parameter "display as" box several times and all of them will be set as a change-disabled default value. steps to reproduce: 1.new a parameter display type"List Box". checkbox "allow null value" 2.In "Display As" box of parameter, place cursor on first row.Input nothing. 3.Switch to second row. then place the cursor on the third row. 4.repeat step 3~~~ Actual result: Refer to the screenshot, several default null value is set.And the default value can't be removed. Expeted result: Only one "null" is allowed.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
private static final String FLAG_DEFAULT = Messages.getString( "ParameterDialog.Flag.Default" ); 
	private static final String COLUMN_VALUE = Messages.getString( "ParameterDialog.Column.Value" ); 
	private static final String COLUMN_DISPLAY_TEXT = Messages.getString( "ParameterDialog.Column.DisplayText" ); 
	private static final String COLUMN_IS_DEFAULT = Messages.getString( "ParameterDialog.Column.Default" ); 
	private static final String BOOLEAN_TRUE = Messages.getString( "ParameterDialog.Boolean.True" ); 
	private static final String BOOLEAN_FALSE = Messages.getString( "ParameterDialog.Boolean.False" ); 
	private static final String PARAM_CONTROL_LIST = DesignChoiceConstants.PARAM_CONTROL_LIST_BOX
			+ "/List"; 
	private static final String PARAM_CONTROL_COMBO = DesignChoiceConstants.PARAM_CONTROL_LIST_BOX
			+ "/Combo"; 
	private static final String DISPLAY_NAME_CONTROL_LIST = Messages.getString( "ParameterDialog.DisplayLabel.List" ); 
	private static final String DISPLAY_NAME_CONTROL_COMBO = Messages.getString( "ParameterDialog.DisplayLabel.Combo" ); 
	private static final String NONE_DISPLAY_TEXT = Messages.getString( "ParameterDialog.Label.None" ); 
	private static final Image DEFAULT_ICON = ReportPlatformUIImages.getImage( IReportGraphicConstants.ICON_DEFAULT );
	private static final Image ERROR_ICON = ReportPlatformUIImages.getImage( ISharedImages.IMG_OBJS_ERROR_TSK );
	private static final String STANDARD_DATE_TIME_PATTERN = "MM/dd/yyyy hh:mm:ss a"; 	
	private HashMap dirtyProperties = new HashMap( 5 );
	private ArrayList choiceList = new ArrayList( );
	private static IChoiceSet dataType = DesignEngine.getMetaDataDictionary( )
			.getChoiceSet( DesignChoiceConstants.CHOICE_PARAM_TYPE );
	private static IChoiceSet controlType = DesignEngine.getMetaDataDictionary( )
			.getChoiceSet( DesignChoiceConstants.CHOICE_PARAM_CONTROL );
	private ScalarParameterHandle inputParameter;
	private boolean loading = true;
	private static final SelectionChoice dummyChoice = StructureFactory.createSelectionChoice( );
	private Text nameEditor, promptTextEditor, defaultValueEditor,
			helpTextEditor, formatField;
	
	private Label promptMessageLine;