usb-creator continues installing after cancel selected

[Paul Worrall]

When writing an iso to USB stick, I click 'Cancel' on the progress bar dialog, and then click 'Yes' on the 'Do you really want to quit' dialog but the installation still continues silently in the background

There is still a usb-creator-kde process running, and dbus-monitor --system shows USBCreator Progress signals still generated. Eventually it finishes and the Device Notifier pops up to confirm the new device is ready.

Using usb-creator-kde from Kubuntu 18.04 LTS
paul@desktop:~$ sudo apt-cache policy usb-creator-kde
usb-creator-kde:
  Installed: 0.3.5
  Candidate: 0.3.5
  Version table:
 *** 0.3.5 500
        500 http://mirror.ox.ac.uk/sites/archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        100 /var/lib/dpkg/status