built_value remains one of the most popular source-generation packages in the Flutter ecosystem. It is powerful, but it can also be difficult to work with, especially when the error message does not clearly explain the real problem.
Deserializing failed due to Error: Bad State No element is one of those messages. It is vague, and I once spent an entire day debugging it. In practice, this error often comes from incorrect field types in your model class.
In particular, the class may contain a field typed as dynamic, either explicitly or implicitly. Common examples are bare List and Map declarations without concrete type parameters.
The fix is to make the types explicit, for example List<String> or Map<String, String>. If the field is not needed, removing it is also a valid solution.
📚 Hope you enjoy reading!