Steps top reproduce:
 In a clean version of DXP, add config for additional CSS classes for heading element as shown in https://doc.ibexa.co/en/latest/content_management/rich_text/extend_online_editor/#custom-css-classes

Pay attention to use multiple choices with required set to false as below:

ibexa:
    system:
        admin_group:
            fieldtypes:
                ezrichtext:
                    classes:
                        heading:
                            choices: [ regular, special, tip]
                            required: false
                            multiple: true

You will get an error: It turns out that a default_value is required to make it work despite of the required set to false

Expected:

The config above should be working