'NoneType' object is not subscriptable when serializing fields that do not belong to the database is None

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```py
class UserConfig(SQLModel, table=True):
__tablename__ = "user_config"
id: Optional[int] = Field(default=None, primary_key=True)
user_id: Optional[int]
_user_name: str = '' #---> not in database fields

@property
def user_name(self):
return self._user_name

@user_name.setter
def user_name(self, user_name):
self._user_name = user_name

class UserConfigSchema(Schema):
user_id = fields.Int()
user_name = fields.Str(allow_none= True)
id = fields.Int()

class Meta:
model = UserConfig
unknown = EXCLUDE
```

user_config_schema.dumps(user_config) # --->issue NoneType .....
what i should to do in that time.
Please confirm whether it needs to be repaired or if there are any solutions other than @post_dump

Start by reproducing the failure with the shown UserConfig, UserConfigSchema, and user_config_schema.dumps(user_config) example. Trace how the schema resolves the user_name property when serializing a field absent from the database, then establish whether the desired outcome is successful serialization or documented use of a workaround and verify it with a regression test.

Written by the indexing model from the issue text.