django-safedelete: cannot unpack non-iterable NoneType object with `SOFT_DELETE_CASCADE`

I have a parent child model and i want if the parent object got deleted all the children get deleted too (safe delete in my case):

class Review(SafeDeleteModel):
    _safedelete_policy = SOFT_DELETE_CASCADE
    parent = models.ForeignKey(
        'self',
        verbose_name=_("Parent"),
        related_name="children",
        on_delete=models.CASCADE,
        null=True,
        blank=True
    )

and if i delete the parent object this error will be raised: cannot unpack non-iterable NoneType object