state_pickler.get_state fails for attributes resulting in StateTuple

[johannesloibl]

from traits.api import HasTraits, Dict, Str, Tuple, List
from apptools.persistence import state_pickler as sp


class Model(HasTraits):
    dict_item = Dict()
    tuple_item = Tuple()
    str_item = Str()
    list_item = List()


def main():
    inner = Model(
        dict_item=dict(a=1),
        tuple_item=(1, 2, 3),
        str_item="bla",
        list_item=[4, 5, 6]
    )
    outer = Model(
        dict_item=dict(a=inner),
        tuple_item=(inner, inner),
        str_item="blubb",
        list_item=[inner, inner]
    )

    state = sp.get_state(outer)


if __name__ == '__main__':
    main()

This results in following exception:

Removing tuple_item=(inner, inner), makes it work again.

OS: Win 10
Pkg Versions:
traits==6.3.1
apptools==5.1.0