Steps to reproduce:

1. Create a new custom Page Builder block:

   ibexa_fieldtype_page:
       blocks:
           event:
               views:
                   default:
                       template: '@ibexadesign/blocks/event/template.html.twig'
                       name: Default view
                       priority: -255
               name: Event Block
               category: Custom
               thumbnail: /bundles/ibexaicons/img/all-icons.svg#date
               attributes:
                   name:
                       type: text
                       name: Event name
                       validators:
                           not_blank:
                               message: Please provide a name
                   category:
                       type: select
                       name: Select a category
                       value: visual
                       options:
                           multiple: true
                           choices:
                               'Music': music
                               'Visual arts': visual
                               'Sports': sports
                           help:
                               text: '<h3>Overwrites page setting for this block.</h3>'
                               html: true
   

2. Add this block in a Landing Page and then try to configure this block inside Page Builder.

Expected result:
One is presented with an exception:

An error has occurred resolving the options of the form "Ibexa\FieldTypePage\Form\Type\BlockAttribute\AttributeSelectType": The option "help" with value array is expected to be of type "string" or "null" or "Symfony\Component\Translation\TranslatableMessage", but is of type "array".

Result:
The block should be configurable without an issue.