Admin UI: Assign product to channel during creation

[CamTosh]

Describe the bug
It is not possible to add a product to a channel before it has been created. But the interface offers an "ASSIGN TO CHANNEL" button which leads to the error "Either the Product id or slug must be provided" (slug i18: product-id-or-slug-must-be-provided)

To Reproduce
Steps to reproduce the behavior:

1. Go to '/admin/catalog/products/create' (stay on "default channel")
2. Fill new product info
3. Click on 'Assign to channel'
4. See error Either the Product id or slug must be provided

Expected behavior
The button should not be available until the product is created.

Environment (please complete the following information):

• @vendure/core version: 1.1.5
• Nodejs version: 14.16.1
• Database: Postgres

Additional context