JENKINS-16869: I can't reproduce the problem reported, but I do see that
you can see the timestamper configuration slicer even when the
timestamper plugin is not installed, which gives you this error. I've
made a change so that the button is not enabled if the plugin is not
available.
Merge pull request #10 from ninian/master

FIXED JENKINS-16869: Exception when clicking on timestamper link