mainwindow: fix wrong account selected in combo box in some cases

When account is at the top of the list in the account combo box and
it is disabled, quitting the application and relaunching it results
in that disabled account being wrongly selected in the combo box.
This fixes that.

GitLab: #1212
Change-Id: If286655ff61e50d2d1c6c4a24a66231f5aef70c3