Preconditions:

• Added Attribute group
• Added few Attributes based on Attribute group

Steps to reproduce:

1. Log in to back office.
2. Go to Product catalog / Product types.
3. Start creating a new PT.
4. Add Attribute group with defined Attributes to Attribute section by drag and drop.
5. Turn on 'Required field' or 'Used for product variants' for one of attribute.
6. Save the form.

Actual result:

See attached screenshot.

An error 500 occurred while 'required field' or 'used for product variants' is turned on for defined attributes.

Argument 1 passed to Ibexa\ProductCatalog\Local\Persistence\Cache\AttributeDefinition\Handler::loadByIdentifier() must be of the type string, null given, called in /Users/bogdanmazur/flex/flexCommerceMaster44dev/vendor/ibexa/product-catalog/src/lib/FieldType/ProductSpecification/Persistence/Gateway/FieldConstraintsStorage.php on line 87 

Expected result:

Properly added product type with defined attributes.

Note:
Issue doesn't occur on the last released tag v4.4.1