RelatedDocsPlugin tries to create node at wrong moment

The RelatedDocsPlugin tries to create a subnode "relateddocs:docs" in the constructor if it's not there, i.e. if not provided by document prototype. The constructor is the wrong location for this, or at least some condition should be added, as in the document editor context, a published document node is not checked out which it must be to be able to add nodes.

It leads to the document not being shown (an empty tab) and a VersionException: cannot add a child to a checked-in node.