Validation doesn't work for Combobox

I have a big problem with validation in combobox. It is created by FieldFactory. And validation ( I mean required validation) doesn’t work for this field.
It’s a little strange because the validation works when if you set something and return empty value back. But in initial state not working.

If I understood the property will validate If It was registered.

Form.class

    public boolean addItemProperty(Object id, Property property) {

        // Checks inputs
        if (id == null || property == null) {
            throw new NullPointerException("Id and property must be non-null");
        }

        // Checks that the property id is not reserved
        if (propertyIds.contains(id)) {
            return false;
        }

        propertyIds.add(id);
        ownProperties.put(id, property);

        // Gets suitable field
        final Field field = fieldFactory.createField(this, id, this);
        if (field == null) {
            return false;
        }

        // Configures the field
        field.setPropertyDataSource(property);

        // Register and attach the created field
        addField(id, field);

        return true;
    }

public void addField(Object propertyId, Field field) {
        registerField(propertyId, field);
        attachField(propertyId, field);
        requestRepaint();
    }

Source code shows that combobox field will be registered anyway.
Where do I have mistakes?