Steps to reproduce

1.  Execute edit action on "Product detail" component
2.  Click on the "cross" button for removing link field value
3.  Exception is thrown see attached stacktrace.rtf file.

Expected result

Do not throw an exception

Code snippet exception 

Caused by: java.lang.NullPointerException

at info.magnolia.ecommerce.app.catalogs.chooser.CategorySelectFieldSupport$1.convertToModel(CategorySelectFieldSupport.java:66) ~[magnolia-ecommerce-ui-1.2.4-SNAPSHOT.jar:?]

at info.magnolia.ecommerce.app.catalogs.chooser.CategorySelectFieldSupport$1.convertToModel(CategorySelectFieldSupport.java:63) ~[magnolia-ecommerce-ui-1.2.4-SNAPSHOT.jar:?]

at com.vaadin.data.Converter$2.lambda$convertToModel$5daf174e$1(Converter.java:167) ~[vaadin-server-8.12.4.jar:8.12.4]