ERROR: Invalid URL is shown after adding a new group to "User Group"

Example:

1. Go to "Administration" -> "Users" -> "User Group
2. Open an existing User Group
3. Click on "Add" under read-only
4. Select a host group to add
5. Pink / Red message pops up saying: ERROR: Invalid URL
6. Close popup window with error in it
7. Group has been added even though error was thrown.