muon updater unresponsive after cancel

If I click "Install updates" and then click "Cancel" at the password prompt, the "Install updates" button is no longer responsive and I have to exit the program and then re-open it. On one occasion, when testing, the program stayed in the "Waiting for authentication" state and had to be xkilled.

I would expect the program to return to the same state as when it is first opened - the user may have clicked "Cancel" simply to check something about one of the packages to be updated - and to be ready to proceed again with the update.

kubuntu 15.04 - 64bit

$lsb_release -rd
Description: Ubuntu 15.04
Release: 15.04

$ apt-cache policy muon
muon:
  Installed: (none)
  Candidate: 4:5.2.2-1-0ubuntu4
  Version table:
     4:5.2.2-1-0ubuntu4 0
        500 http://ie.archive.ubuntu.com/ubuntu/ vivid/universe amd64 Packages