Related Entities example fails for "Customer" property

When using the [Related Entities example](https://demo.jmix.io/sampler/#main/0/sample?id=relatedentities-simple) an error occurs when using the "Customer" property.

See attached screencast.

![related-entities-customer-error](https://user-images.githubusercontent.com/817400/127199431-db9c172d-1545-4d5d-ac64-22a491f6e7d4.gif)