Multi user task instance not working with persistence

I have a sample of this at https://github.com/yasermalik/kogitoSample .

 When I try to use a Multi-instance user task node, and I provide it a list class which I have extended from ArrayList, I get this error when trying to build the app

`Error during processing model classes: Error while generating proto for model class class com.masergy.workflow.dto.myList Impossible to find field empty in class java.util.AbstractCollection`

Creating a bug ticket as tdolphin-1 asked me to in zulip chat.