See https://git.magnolia-cms.com/projects/MODULES/repos/magnolia-content-types/pull-requests/3/overview
Jackrabbit throws exception if you try to register nodetypes with yet unknown namespace.
Namespaces need to be register with an URI. What URI should be used?
Besides the exception that is thrown is swallowed. That should definitely not be the case. I'll create a follow up, as we need this commit for the others to apply.

info.magnolia.repository.DefaultRepositoryManager#registerNameSpacesAndNodeTypes
info.magnolia.jackrabbit.ProviderImpl#registerNamespace
javax.jcr.NamespaceRegistry#registerNamespace