window: Add null-check for list_pane on startup
To implement search-as-you-type, we make sure to focus the search field if that wasn't the case already. However, if a user types something wile Contacts is still starting up, the UI hasn't finished by that time already, so that will lead us to dereferencing a NULL-pointer.
To fix this nice and easy, let's to do a quick NULL-check before we try to focus anything.
Fixes: #155 (closed)