Fixes #1818: InventoryItem API serializer no longer requires specifying a null value for items with no parent

netbox/dcim/api/serializers.py

@@ -733,6 +733,8 @@ class InventoryItemSerializer(serializers.ModelSerializer):
 
 
 class WritableInventoryItemSerializer(ValidatedModelSerializer):
+    # Provide a default value to satisfy UniqueTogetherValidator
+    parent = serializers.PrimaryKeyRelatedField(queryset=InventoryItem.objects.all(), allow_null=True, default=None)
 
     class Meta:
         model = InventoryItem