ArticleListWidget::handleArticleList() can be called inside
ArticleListWidget::handleArticleAboutToBeRemoved() and list widget
item can be removed at this point. Now we checking for it existence.
Closes#6896.
Revert back old behavior (incorrect but, perhaps, more user-friendly),
when Unread list populates with "unread" articles only but then it can
contain "read" articles if they become "read" when Unread list is shown.