Fix error when deleting a vocabulary entry whose key contains a backslash

Steps to reproduce

1. Go to Administration > Vocabularies and select "continent"
2. Click on the button Add Entry
3. Set the properties with

   ID = At\antis
   Label = Atlantis
   

4. Click on the Delete icon for the newly created entry

Observe an error message is displayed which indicates that a problem occurred when deleting the entry

 
Expected behavior:

The entry is deleted without error