Unable to use UDW for image in block:

1. create new custom block:

ezplatform_page_fieldtype:
    blocks:
        hero:
            name: Hero
            category: default
            thumbnail: 'assets/images/page_builder/blocks/hero.svg'
            views:
                default: { name: 'Default block layout', template: '@ezdesign/blocks/hero.html.twig' }
            attributes:
                text:
                    name: Content
                    type: text
                    validators:
                        not_blank:
                            message: You must provide a text
                        regexp:
                            options:
                                pattern: '/[^\\s]/'
                            message: Only valid HTML code is allowed
                contentId:
                    name: Image
                    type: embed
                    validators:
                        not_blank:
                            message: You must provide value
                        regexp:
                            options:
                                pattern: '/[0-9]+/'
                            message: Choose a Content item

2. goto edit mode in page builder
3. drag&drop new block, go to configuration for new block
4. try to click on 'select content' for image field.

Expected behaviour: UDW should show up
Current behaviour: nothing happened, error when trying to save the block configuration:
`An invalid form control with name='block_configuration[attributes][image][value]' is not focusable.`