Cannot add new product with product variants because an error 500 occurred

Preconditions:

• Clean installation of Commerce on branch 2.5 using PHP 7.4

Steps to reproduce:

1. Log in to backoffice.
2. Go to Content / Content structure.
3. Start creating a new content 'Product' with product variant e.g. 'Size'.
4. Publish product.

Actual result:

See attached screenshots.

Cannot add new product with product variants because an error 500 occurred

Notice: Trying to access array offset on value of type null

Expected result:

Product with product variant / variants can be publish and can be open on FT site.