[PHP8] Type mixed cannot be marked as nullable since mixed already includes null

[bcorcoran]

{
    "message": "Type mixed cannot be marked as nullable since mixed already includes null",
    "exception": "Symfony\\Component\\ErrorHandler\\Error\\FatalError",
    "file": "/Users/USERNAME/Sites/WEBSITE/vendor/jackabox/nova-duplicate-field/src/DuplicateField.php",
    "line": 17,
    "trace": []
}