DestroyUI causes an error message in firefox

Calling DestroyUI (closing any form) cause the following mesage to appear in the console:

May be this is related to the fact that the DestroyUI is a GET request and it does not return anything? It is also a little strange that it is a GET request because it appears to change state on the server.

This message only pops up in firefox. But even there everything seem sto work just fine. So the problem is just the message appearing.