Created Custom URL Aliases for Form can't be deleted (trash button stays grayed out).

Steps to reproduce:
1. Clean installation of eZ Platform EE 2.5.6
2. Log into the BackOffice.
3. In Forms tab create a new Form: Test Form. Publish it.
4. In Test Form URL tab click on + button to add Custom URL Alias.
5. Fill all required fields and push the Create button.
6. On the list of Custom URL Aliases for Test Form click on the checkbox next to previously created URL Alias.

Result
The trash button stays grayed out (disabled).

Expected Results
Trash button is no longer grayed out and you can delete Custom URL Alias.