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
createDataSet.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				new NewDataSetAction( ).run( );
				refreshDataSets( );
			}
		} );
		createLabel( composite, LABEL_SELECT_VALUE_COLUMN );
		
		columnChooser = new Combo( composite, SWT.BORDER | SWT.DROP_DOWN );
		columnChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		columnChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetDefaultSelected( SelectionEvent e )
			{
				updateButtons( );
			}
		} );
		Button valueColumnExprButton = new Button( composite, SWT.PUSH );
		valueColumnExprButton.setText( "..." ); 
		valueColumnExprButton.setToolTipText( Messages.getString( "ParameterDialog.toolTipText.OpenExprButton" ) );
		valueColumnExprButton.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent event )
			{
				ExpressionBuilder expressionBuilder = new ExpressionBuilder( getExpression( columnChooser.getText( ) ) );
				expressionBuilder.setExpressionProvier( new ParameterExpressionProvider( inputParameter,
						dataSetChooser.getText( ) ) );
				if ( expressionBuilder.open( ) == OK )
				{
					setExpression( columnChooser, expressionBuilder.getResult( )
							.trim( ) );