Error while trying to delete a contact

Bug description

Error while trying to delete a contact.

Steps to reproduce

The contact does not exist anymore on the ejabberd server.

Right click on the contact > Manage Contact > Remove > Validate

    Traceback (most recent call last):
      File "src\dialogs.py", line 1746, in on_response_ok
      File "src\roster_window.py", line 3671, in on_ok
      File "src\common\connection.py", line 2251, in unsubscribe
    AttributeError: 'NoneType' object has no attribute 'delItem'

Software versions

OS version: Windows 8.1

GTK version: 2.24.10

PyGTK version: 2.24.0