Generally this works and has been done several times, but this time there was a problem:
In https://apps.top-logic.com/contract-dev the class tl.contract.core:Contract should be dragged via drag/drop into the diagram of the module tl.contract:offer to link it there with the class tl.contract:offer:Offer.
After the "drop" there was an error message and the representations of both classes overlapped each other. The process was repeated several times with always the same result.
No test. Not reproducible.