parler with mptt

[digitalbim]

str returned non-string (type NoneType)
when choosing to enter another language in admin.

Step to reproduce:

In admin you can select one of the language tabs: If you fill in one then save - No problems /Errors. When you then go back into the entry to add another language it causes the no string type error.

Model

class MainMenu(MPTTModel, TranslatableModel):
"""
Model defines the main menu for the application
"""
id = models.UUIDField(
    primary_key=True,
    default=uuid.uuid4,
    editable=False
)

parent = TreeForeignKey(
    "self",
    on_delete=models.CASCADE,
    null=True,
    blank=True,
    related_name='child'
)

translations = TranslatedFields(
    name=models.CharField(
        max_length=50,
        help_text="Enter the display name",
        verbose_name="name",
        blank=True,
        null=True
    ),
    slug=models.SlugField(
        max_length=200,
        db_index=True),



)

icon = models.CharField(
    max_length=40,
    help_text="Enter the icon name",
    verbose_name="Display Icon"
)
url = models.CharField(
    max_length=255,
    help_text="Enter the url name",
    verbose_name="Url"
)
order = models.IntegerField(
    verbose_name="Menu order",
    help_text="Choose menu order",
)

class MPTTMeta:
    order_insertion_by = ['order']

class Meta:
    verbose_name = "Menu item"

def __str__(self):
    return self.safe_translation_getter('name', any_language=True)

objects = NavigationManager()

Manager:

class NavigationQuerySet(TranslatableQuerySet, TreeQuerySet):

def as_manager(cls):
    # make sure creating managers from querysets works.
    manager = NavigationManager.from_queryset(cls)()
    manager._built_with_as_manager = True
    return manager
as_manager.queryset_only = True
as_manager = classmethod(as_manager)

class NavigationManager(TreeManager, TranslatableManager):
     _queryset_class = NavigationQuerySet

Admin:

class NavigationAdminForm(MPTTAdminForm, TranslatableModelForm):
     pass


class NavigationAdmin(TranslatableAdmin, MPTTModelAdmin):
    form = NavigationAdminForm

    def get_prepopulated_fields(self, request, obj=None):
        return {'slug': ('name',)}  # needed for translated fields


admin.site.register(MainMenu, NavigationAdmin)