When I add a new paragraph in the empty richtext and I assign a custom class to it then I have a javascript error.

Steps to reproduce:

1. Add custom classes to paragraph in your config

ibexa:
    system:
        admin_group:
            fieldtypes:
                ezrichtext:
                    classes:
                        paragraph:
                            choices: [paragraph1, paragraph2, paragraph3]
                            default_value: paragraph1
                            required: false
                            multiple: false 

1. Create or edit content with richtext attribute
2. Add new paragraph and assign a custom class
3. In the console you will see the error (screenshot above)

The error doesn't seem to break anything, the class is assigned anyway.