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 LABEL_SELECT_DISPLAY_TEXT = Messages.getString( "ParameterDialog.Label.SelectDisplayText" ); 
	private static final String LABEL_SELECT_VALUE_COLUMN = Messages.getString( "ParameterDialog.Label.SelectValueColumn" ); 
	private static final String LABEL_SELECT_DATA_SET = Messages.getString( "ParameterDialog.Label.SelectDataSet" ); 
	private static final String LABEL_PREVIEW = Messages.getString( "ParameterDialog.Label.Preview" ); 
	private static final String CHECKBOX_ALLOW_NULL = Messages.getString( "ParameterDialog.CheckBox.AllowNull" ); 
	private static final String CHECKBOX_ALLOW_BLANK = Messages.getString( "ParameterDialog.CheckBox.AllowBlank" ); 
	private static final String CHECKBOX_DO_NOT_ECHO = Messages.getString( "ParameterDialog.CheckBox.DoNotEchoInput" ); 
	private static final String CHECKBOX_SORT = Messages.getString( "ParameterDialog.CheckBox.Sort" ); 
	private static final String CHECKBOX_HIDDEN = Messages.getString( "ParameterDialog.CheckBox.Hidden" ); 
	private static final String BUTTON_LABEL_CHANGE_FORMAT = Messages.getString( "ParameterDialog.Button.ChangeFormat" ); 
	private static final String BUTTON_LABEL_IMPORT = Messages.getString( "ParameterDialog.Button.ImportValue" ); 
	private static final String BUTTON_LABEL_SET_DEFAULT = Messages.getString( "ParameterDialog.Button.SetDefault" ); 
	private static final String BUTTON_LABEL_REMOVE_DEFAULT = Messages.getString( "ParameterDialog.Button.RemoveDefault" ); 
	private static final String BUTTON_LABEL_DELETE = Messages.getString( "ParameterDialog.Button.Delete" ); 
	private static final String BUTTON_CREATE_DATA_SET = Messages.getString( "ParameterDialog.Button.CreateDataSet" ); 
	private static final String RADIO_DYNAMIC = Messages.getString( "ParameterDialog.Radio.Dynamic" ); 
	private static final String RADIO_STATIC = Messages.getString( "ParameterDialog.Radio.Static" ); 
	private static final String INPUT_PROPMT = Messages.getString( "ParameterDialog.Prompt.Input" ); 
	private static final String ERROR_TITLE_INVALID_INPUT = Messages.getString( "ParameterDialog.ErrorTitle.InvalidInput" ); 
	private static final String ERROR_TITLE_INVALID_LIST_LIMIT = Messages.getString( "ParameterDialog.ErrorTitle.InvalidListLimit" ); 
	private static final String ERROR_MSG_CANNOT_BE_BLANK = Messages.getString( "ParameterDialog.ErrorMessage.CanootBeBlank" ); 
	private static final String ERROR_MSG_CANNOT_BE_NULL = Messages.getString( "ParameterDialog.ErrorMessage.CanootBeNull" ); 
	private static final String ERROR_MSG_DUPLICATED_VALUE = Messages.getString( "ParameterDialog.ErrorMessage.DuplicatedValue" ); 
	private static final String ERROR_MSG_DUPLICATED_LABEL = Messages.getString( "ParameterDialog.ErrorMessage.DuplicatedLabel" ); 
	private static final String ERROR_MSG_MISMATCH_DATA_TYPE = Messages.getString( "ParameterDialog.ErrorMessage.MismatchDataType" ); 
	private static final String ERROR_MSG_DUPLICATED_NAME = Messages.getString( "ParameterDialog.ErrorMessage.DuplicatedName" ); 
	private static final String ERROR_MSG_NAME_IS_EMPTY = Messages.getString( "ParameterDialog.Errormessage.EmptyName" ); 
	private static final String ERROR_MSG_NO_DEFAULT_VALUE = Messages.getString( "ParameterDialog.ErrorMessage.NoDefaultValue" ); 
	private static final String ERROR_MSG_NO_AVAILABLE_COLUMN = Messages.getString( "ParameterDialog.ErrorMessage.NoAvailableColumn" ); 
	private static final String ERROR_MSG_INVALID_LIST_LIMIT = Messages.getString( "ParameterDialog.ErrorMessage.InvalidListLimit" );