The S3CopyArtifact configuration is supposed to have a selector parameter but the dropdown is not rendered. In the end, the configuration is stored with a null which create a NullPointerException during the build as the null is not tested.

<ca:selectorList currentSelector="${instance.buildSelector}"
      name="selector" title="Which build"/>

is the UI code which should offer the BuildSelector (from copyartifact-plugin) dropdown. But the xmlns:ca is supposed to be in /hudson/plugins/copyartifact but is in fact in the s3 plugin.