Apache Flink

The error I'm running into is:
`Caused by: org.apache.flink.streaming.runtime.tasks.StreamTaskException: Could not serialize object for key serializedUDF.`

And I _believe_ it is caused by the inner class Py4J requires to register as an interface callback.

eg: this bit-
```class DictElementConverter:
    class Java:
        implements = ['org.apache.flink.connector.base.sink.writer.ElementConverter',
                      'java.io.Serializable']```

Without setting that inner class, I get the following error:
```AttributeError: 'DictElementConverter' object has no attribute '_get_object_id'```

What happens if you subclass DictElementConverter itself and have it implement serializable? Does the first error go away?