Django admin error ValidationError: ['ManagementForm data is missing or has been tampered with'] due to conditional inline use

Question

I have a custom User model(AbstractUser) and a Stock model. Assume there's multiple user roles manager, supplier etc. The manager can be able to view other manager's details and supplier details.

The User model got a conditional inline which shows the stocks of each supplier if the user role is equalled to supplier.(Here role is a PositiveSmallIntegerField with choices and SUPPLIER = 2)

class SupplierStockInline(admin.StackedInline):
    """ Inline to show stocks of a supplier """

    model = Stock
    extra = 0


@admin.register(User)
class UserAdmin(UserAdmin):
    """ User """

    fieldsets = [
        (None, {'fields': ('username', 'password')}),
        ('Personal info', {'fields': (
            'first_name',
            'last_name',
            'email',
            ... some custom fields...
        )}),
        ('Permissions', {'fields': (
            'is_active',
            'is_staff',
            'is_superuser',
            'role',
            'groups',
            'user_permissions',
        )}),
        ('Important dates', {'fields': ('last_login', 'date_joined')})
    ]

    list_display = [...]
    search_fields = [...]

    # --- the source of the problem ---

    inlines = []

    def get_inlines(self, request, obj):
        """ Show inlines, stocks of supplier """
        try:
            if obj.role == SUPPLIER:
                return [SupplierStockInline]
        except:
            pass
        return []
    # --- ---- -----

This works just fine till I tried to change the role of a new user to supplier.

ValidationError: ['ManagementForm data is missing or has been tampered with']

The issue is due to that overridden get_inlines() method. When I comment out the get_inlines() it works fine and this is only happening for the role supplier. I tried to tackle this but unabled to come up with a solution.

Hoping for guidance to solve the issue, thanks in advance.