Request failed with status code 500

[pfuju]

Describe the bug
If I choose [advanced option enabled] "map to an existing table" (e.g.: fe_users) I got an 500 error and a html stacktrace in the modal window.

Exception: EBT\ExtensionBuilder\Domain\Validator\ExtensionValidator:: validationResult()
This means the type field can not be used for defining the record type.
You have to configure the mappings yourself if you want to map to this.

To Reproduce
Steps to reproduce the behavior:

1. Create a new model
2. Select "advanced option enabled" and set the field "map to an existing table" to "fe_users"
3. See error

Expected behavior
In previous versions it was possible to map the Extbase Model to an existing table (e.g. fe_users, tt_address) and extend it.

TYPO3 Version
Using TYPO3 12.4.17

Extension Builder Version:
dev-main (v12-react)

[pfuju]

May be related to #787