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
dataTypeChooser = new Combo( propertiesSection, SWT.READ_ONLY
				| SWT.DROP_DOWN );
		dataTypeChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		dataTypeChooser.setItems( ChoiceSetFactory.getDisplayNamefromChoiceSet( dataType ) );
		dataTypeChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				changeDataType( );
				if ( allowBlank != null && !allowBlank.isDisposed( ) )
				{
					if ( DesignChoiceConstants.PARAM_TYPE_STRING.equals( getSelectedDataType( ) ) )
					{
						allowBlank.setEnabled( true );
					}
					else
					{
						allowBlank.setEnabled( false );
						allowBlank.setSelection( false );
						checkBoxChange( allowBlank, CHECKBOX_ALLOW_BLANK );
					}
				}
			}
		} );
		createLabel( propertiesSection, LABEL_DISPALY_TYPE );
		controlTypeChooser = new Combo( propertiesSection, SWT.READ_ONLY
				| SWT.DROP_DOWN );
		controlTypeChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		controlTypeChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{