Don't crash when removing last account

Summary:
Under certain circumstances, removing the last (or only) remaining account
causes kube to crash.

Steps to reproduce:

1. clear all sink config folders
2. start kube, add a generic IMAP account
3. close kube, reopen kube
4. open accounts list, select "edit"
5. Select "Delete" -> kube crashes

Reviewers: Kube, cmollekopf, aacid

Reviewed By: Kube, cmollekopf

Subscribers: aacid, cmollekopf

Differential Revision: https://phabricator.kde.org/D4517