Bug 182673 No default value error message prompts incorrectly
Steps to reproduce: 1. New a report parameter; 2. Choose control type as Combo; 3. Choose parameter as dynamic type; Expected result: As the isRequired is false by default, parameter's default value can be null value. There should be no error message about no default value prompt. Actual result; Error messages prompts
UI/org.eclipse.birt.report.designer.ui/src/org/eclipse/birt/report/designer/ui/dialogs/ParameterDialog.java
createLabel( valueArea, LABEL_DEFAULT_VALUE );
		defaultValueChooser = new Combo( valueArea, SWT.BORDER );
		defaultValueChooser.setLayoutData( new GridData( GridData.FILL_HORIZONTAL ) );
		if ( !isStatic( ) )
		{
			defaultValueChooser.add( CHOICE_SELECT_VALUE );
		}
		if ( getSelectedDataType( ).equals( DesignChoiceConstants.PARAM_TYPE_STRING )
				&& !isRequired.getSelection( ) )
		{
			defaultValueChooser.add( CHOICE_NULL_VALUE );
			defaultValueChooser.add( CHOICE_BLANK_VALUE );
		}
		defaultValueChooser.addSelectionListener( new SelectionAdapter( ) {
			public void widgetSelected( SelectionEvent e )
			{
				if ( defaultValueChooser.getSelectionIndex( ) == -1 )
					return;
				String selection = defaultValueChooser.getItem( defaultValueChooser.getSelectionIndex( ) );
				if ( selection.equals( CHOICE_SELECT_VALUE ) )
				{
					defaultValueChooser.setText( "" );
					if ( getColumnValueList( ).isEmpty( ) )
						return;
					SelectParameterDefaultValueDialog dialog = new SelectParameterDefaultValueDialog( Display.getCurrent( )
							.getActiveShell( ),
							Messages.getString( "SelectParameterDefaultValueDialog.Title" ) );
					dialog.setColumnValueList( getColumnValueList( ) );
					int status = dialog.open( );
					if ( status == Window.OK )