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
import org.eclipse.swt.widgets.Control;
import org.eclipse.swt.widgets.Group;
import org.eclipse.swt.widgets.Item;
import org.eclipse.swt.widgets.Label;
import org.eclipse.swt.widgets.Shell;
import org.eclipse.swt.widgets.Table;
import org.eclipse.swt.widgets.TableColumn;
import org.eclipse.swt.widgets.Text;
import org.eclipse.ui.ISharedImages;
import com.ibm.icu.util.ULocale;
/**
 * The dialog used to create or edit a parameter
 */
public class ParameterDialog extends BaseDialog
{
	private static final String CHOICE_NO_DEFAULT = Messages.getString( "ParameterDialog.Choice.NoDefault" ); 
	private static final String GROUP_PROPERTIES = Messages.getString( "ParameterDialog.Group.Properties" ); 
	private static final String GROUP_MORE_OPTION = Messages.getString( "ParameterDialog.Group.MoreOption" ); 
	private static final String LABEL_NAME = Messages.getString( "ParameterDialog.Label.Name" ); 
	private static final String LABEL_DATETIME_PROMPT = Messages.getString( "ParameterDialog.Label.DateTImePrompt" ); 
	private static final String LABEL_PROMPT_TEXT = Messages.getString( "ParameterDialog.Label.PromptText" ); 
	private static final String LABEL_PARAM_DATA_TYPE = Messages.getString( "ParameterDialog.Label.DataType" ); 
	private static final String LABEL_DISPALY_TYPE = Messages.getString( "ParameterDialog.Label.DisplayType" ); 
	private static final String LABEL_DEFAULT_VALUE = Messages.getString( "ParameterDialog.Label.DefaultValue" ); 
	private static final String LABEL_HELP_TEXT = Messages.getString( "ParameterDialog.Label.HelpText" ); 
	private static final String LABEL_LIST_OF_VALUE = Messages.getString( "ParameterDialog.Label.ListOfValue" ); 	
	private static final String LABEL_VALUES = Messages.getString( "ParameterDialog.Label.Value" ); 
	private static final String LABEL_FORMAT = Messages.getString( "ParameterDialog.Label.Format" ); 
	private static final String LABEL_LIST_LIMIT = Messages.getString( "ParameterDialog.Label.Listlimit" );
	private static final String LABEL_NULL = Messages.getString( "ParameterDialog.Label.Null" );