Dev update: QA see simplified steps at the end please.

Steps:

1. I created a new product type with color and measurement attributes https://master-7rqtwti-v5p5smejfqk2u.fr-4.platformsh.site/admin/contenttypegroup/4/contenttype/306/update
   
2. I added "External product data (sesexternaldata)" to Fields of this product type
3. I added a category: Computers & Laptops and a subcategory: Business laptops
4. Then I wanted to create a new product, and assign a subcategory: Business laptops
5. In Create form, "SEO Meta tags" section occurred.
6. I tried to save this product:
   
7. 500 error occurred:
   
8. I clicked "Back" and the error notification with some details was displayed:
   

Simplified steps

On any DXP edition, clean install:

1. Create Product Type with "External product data" (sesexternaldata) Field.
2. Try creating a new product (it's enough to fill name and code)
3. Observe 500 exception (for me it's `Call to a member function getBaseUrl() on null` when handling exception[1])
4. See the last entry in var/log/<your_environment>.log and observe the mentioned error:

   Argument 1 passed to Ibexa\Commerce\FieldTypes\FieldType\SesExternalData\Type::toHash() (...)

[1] An expception when handling an exception seems to be a separate issue.