Magento 2 – Create Order with REST API Without Cart

I'm using REST API with token authentication to export data from a M1 to a fresh installation of M2.2.0 installation.

Everything works fine with customers and products, but I cannot manage to understand how to create an order without creating a cart.

I mean, I already have all needed data from the order, there should be no need to create a cart before, as suggested in all the solutions I found (also in the official magento guide).

I'm following same steps in the above mentioned guide, but trying to post data about the order, I keep getting the error:

"Fatal Error: 'Uncaught Error: Call to a member function getMethodInstance() on null in \/var\/www\/html\/app\/code\/Magento\/Payment\/Observer\/SalesOrderBeforeSaveObserver.php:24\nStack trace:\n#0 \/var\/www\/html\/lib\/internal\/Magento\/Framework\/Event\/Invoker\/InvokerDefault.php(72): Magento\\Payment\\Observer\\SalesOrderBeforeSaveObserver->execute(Object(Magento\\Framework\\Event\\Observer))\n#1 \/var\/www\/html\/lib\/internal\/Magento\/Framework\/Event\/Invoker\/InvokerDefault.php(60): Magento\\Framework\\Event\\Invoker\\InvokerDefault->_callObserverMethod(Object(Magento\\Payment\\Observer\\SalesOrderBeforeSaveObserver), Object(Magento\\Framework\\Event\\Observer))\n#2 \/var\/www\/html\/lib\/internal\/Magento\/Framework\/Event\/Manager.php(66): Magento\\Framework\\Event\\Invoker\\InvokerDefault->dispatch(Array, Object(Magento\\Framework\\Event\\Observer))\n#3 \/var\/www\/html\/generated\/code\/Magento\/Framework\/Event\/Manager\/Proxy.php(95): Magento\\Framework\\Event\\Manager->dispatch('sales_order_sav...', Array)\n#4 \/var\/www\/html\/lib\/internal\/Magento\/Framework\/Model\/AbstractModel.php(694):' in '\/var\/www\/html\/app\/code\/Magento\/Payment\/Observer\/SalesOrderBeforeSaveObserver.php' on line 24"

and this is a sample JSON I'm using to make my tries:

       {
"entity": {
    "customer_email": "provacustomer@mail.com",
    "customer_firstname": "mario",
    "customer_lastname": "rossi",
    "base_grand_total": 120,
    "discount_amount": 0,
    "grand_total": 120,
    "increment_id": "200001414155",
    "shipping_amount": 7,
    "status": "processing",
    "store_id": 1,
    "subtotal": 100,
    "tax_amount": 20,
    "items": {
        "description": "prodotto mio",
        "item_id": 3,
        "price": 10,
        "product_id": 3,
        "product_type": "simple",
        "qty_ordered": 2,
        "sku": "002",
        "tax_amount": 2,
        "weight": 1
    }
},
"billing_address": {
    "firstname": "giuseppa",
    "lastname": "rossi",
    "company": "mycompany",
    "city": "rome",
    "region": "california",
    "region_id": 12,
    "postcode": "12312",
    "telephone": "1234556677",
    "country_id": "US",
    "street": [
        "via prima",
        "via seconda"
    ]
},
"payment": {
    "amount_paid": 120,
    "method": "checkmo"
},
"status_histories": [
    {
        "comment": "new order",
        "status": "processing"
    }
],
"extension_attributes": {
    "shipping_assignments": [
        {
            "shipping": {
                "address": [
                    {
                        "firstname": "franca",
                        "lastname": "vardididi",
                        "company": "mycompany",
                        "city": "rome",
                        "region": "california",
                        "region_id": 12,
                        "postcode": "12312",
                        "telephone": "1234556677",
                        "country_id": "US",
                        "street": [
                            "via prima",
                            "via seconda"
                        ]
                    }
                ],
                "method": "checkmo",
                "total": {
                    "base_shipping_amount": 7
                }
            },
            "items": [
                {
                    "description": "prodotto mio",
                    "item_id": 3,
                    "price": 10,
                    "product_id": 3,
                    "product_type": "simple",
                    "qty_ordered": 2,
                    "sku": "002",
                    "tax_amount": 2,
                    "weight": 1
                }
            ]
        }
    ]
}

}

EDIT

Using the json in this question I managed to create the order.