update-manager won't close properly if it has been closed in "checking for updates" dialog

here is how you repeat the bug:
1. open update-manager
2. press the close button while it checks for updates, the dialog will disappear, then a update-manger will pop-up again asking if we want to update from a previous check
3. press the close button, the update-manger seems to be closed properly
4. run "ps aux | grep update-manager", and we will find that update-manger is still running

release: Ubuntu Utopic Unicorn
update-manger version: 1:14.10.3