Saving a Django model that contains a PolymorphicEmbeddedModelField or PolymorphicEmbeddedModelArrayField with a non-null value raises:

 TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

  This occurs in PolymorphicEmbeddedModelField.get_db_prep_save because self.embedded_models is stored as a list (as passed by the user) but isinstance() requires a tuple as its second argument.

Steps to reproduce:

   class Dog(EmbeddedModel):
      name = models.CharField(max_length=100)

  class Cat(EmbeddedModel):
      name = models.CharField(max_length=100)

  class PetOwner(models.Model):
      pet = PolymorphicEmbeddedModelField([Dog, Cat], null=True)

  PetOwner.objects.create(pet=Dog(name="Rex"))  # raises TypeError

Expected behavior: The instance is saved successfully.

Affected code: django_mongodb_backend/fields/polymorphic_embedded_model.py, get_db_prep_save (two call sites).

I encountered this bug while working on INTPYTHON-771.