Unable to add free form product

[cjsewell]

Attempting to create and save an order containing a free form OrderItem I get the follow error

{
   "errors":{
      "order_line_items.variant":[
         "cannot be attached to a line item as it has been deleted",
         "cannot be attached to a freeform line item"
      ]
   }
}

Extending OrderItem and overriding VariantId making it nullable solved the issue