Cannot edit the defaults of the Openagenda field

Problem/Motivation

It is currently impossible to edit to default value of the field OpenAgenda. When one tries to do so they get the following error :
TypeError: count(): Argument #1 ($value) must be of type Countable|array, string given in count() (line 150 of core/modules/field_ui/src/Form/FieldConfigEditForm.php).

Steps to reproduce

1. Get a barebones Drupal 10 website
2. Enable the OpenAgenda module and the Field UI module
3. Go to the Content Type created by the module (Openagenda)
4. Click on Edit for the field field_openagenda

Proposed resolution

• N/A

Remaining tasks

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User interface changes

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API changes

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Data model changes

• N/A