Bug 166315 Combo box doesn't allow blank when set string type in parameter editor
Combo box doesn't allow blank when set string type in parameter editor Steps to reproduce: 1.New a parameter, display type is Combo Box 2.Select data type as String Expected result: "Allow blank values" is enable. Actual result: "Allow blank values" is gray and can't be checked.
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
{
		clearArea( valueArea );
		lastControlType = null;
		if ( isStatic( ) )
		{
			switchToStatic( );
		}
		else
		{
			switchToDynamic( );
		}
		buildControlTypeList( getSelectedDataType( ) );
		valueArea.layout( );
		initValueArea( );
		updateCheckBoxArea( );
	}
	private void switchToCheckBox( )
	{
		createLabel( valueArea, LABEL_DEFAULT_VALUE );
		defaultValueChooser = new Combo( valueArea, SWT.READ_ONLY | SWT.BORDER );
		defaultValueChooser.add( CHOICE_NO_DEFAULT );
		defaultValueChooser.add( BOOLEAN_TRUE );
		defaultValueChooser.add( BOOLEAN_FALSE );
		defaultValueChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		defaultValueChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				switch ( defaultValueChooser.getSelectionIndex( ) )
				{
					case 0 :