WSOD error when using existing field for "Field with source information"

Problem/Motivation

When trying to use an existing field for "Field with source information", the module throws a White Screen of Death fatal error:

The website encountered an unexpected error. Try again later.

Drupal\Core\Entity\EntityStorageException: 'field_storage_config' entity with ID 'media.field_media_keepeek_1_json' already exists. in Drupal\Core\Entity\EntityStorageBase->doPreSave() (line 519 of core/lib/Drupal/Core/Entity/EntityStorageBase.php).

Drupal creates the media type, but it is missing the *_json field needed for Keepeek.

Steps to reproduce

1. Create a "Keepeek image" media type with these options:
   • Keepeek
   • Create
2. Create a "Keepeek video" media type with these options:
   • Keepeek
   • media.field_media_keepeek
3. Go back to the list of media types and see the media type was created, but look at the list of fields to see that the *_json field needed for Keepeek is missing.

Proposed resolution

When deciding what field machine names to use, make sure the fields don't already exist.

The solution is related to this issue 🐛 Confusing field names when creating 2+ Keepeek media types Active

Remaining tasks

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User interface changes

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