Steps to reproduce

1. Create a product type with a measurement (single) attribute which will be used for variants
2. Create  product based on product type created (1) 
3. Generate variants 
4. Enter Assets and added some images to "All assets"
5. Create an assets collection based on measurement (single) attribute
6. Save it

Actual behaviour

500 error occurred with the following error "Argument 1 passed to Ibexa\Bundle\ProductCatalog\UI\AssetGroup\AssetGroupCollectionFactory::Ibexa\Bundle\ProductCatalog\UI\AssetGroup {closure} () must be of the type string or null, array given" 

Recording showing the case: https://www.awesomescreenshot.com/video/11980123?key=8b6caac6334afa72de16b98d6bac15c3