A user of 1.509.3 with the Metadata plugin formerly installed had some data listed in Discard Old Data screen, as expected:

hudson.model.FreeStyleBuild
jobname #123
CannotResolveClassException: com.sonyericsson.hudson.plugins.metadata.model.MetadataBuildAction

He clicked Discard and received an error:

java.lang.NullPointerException
	at hudson.model.AbstractItem.getRootDir(AbstractItem.java:162)
	at hudson.model.Items.getConfigFile(Items.java:236)
	at hudson.model.AbstractItem.getConfigFile(AbstractItem.java:461)
	at hudson.model.AbstractItem.save(AbstractItem.java:456)
	at hudson.model.Job.save(Job.java:152)
	at hudson.model.AbstractProject.save(AbstractProject.java:268)
	at hudson.diagnosis.OldDataMonitor.doDiscard(OldDataMonitor.java:275)

This means that there is an AbstractProject whose parent is null. Most likely a serialized MetadataBuildAction was persisting a Run which was not identical to the build to which it had been added (this is possible due to lazy-loading), so XStream did not use a backreference but instead tried to serialize the whole build. When deserialized, the orphaned build would have had a bogus parent project.

The solution in 1.519+ (or 1.509.3/4) is simple: use the new RunAction2. There is no alternative available for older versions of Jenkins.