Reproduction:

1.) Install sidebar-link plugin
2.) Use sidebar link in a Build Job "foobar"

Up to that point everything is fine.

3.) Now remove usage of sidebar plugin from the Build Job. Sidebar plugin is still installed!
4.) In config.xml of Build Job "foobar" there's still the empty XML element <hudson.plugins.sidebar__link.ProjectLinks/> included now!
5.) If at any later time from now on Jenkins Job configuration is reloaded or Jenkins is restarted then the whole Build Job is gone (Not being displayed in your Browser any longer!).

The correlated Build Job data still exists on the underlying mass storage device although it is not loaded.

If in config.xml the line containing XML element <hudson.plugins.sidebar__link.ProjectLinks/> is removed then after restart the Job is available again.

Expected behaviour would be that such manual removal of this line shall not be neccessary.

Thank you...