A user of 1.509.3 with the Metadata plugin formerly installed had some data listed in Discard Old Data screen, as expected:
He clicked Discard and received an error:
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.