NPE when requesting content bean in sitemapitem handler

In a sitemapitem handler, called by HstFilter, I try to obtain the current content bean doing this:

HippoBean contentBean = RequestContextProvider.get().getContentBean();

This leads to an NPE in

org.hippoecm.hst.site.request.HstRequestContextImpl#getObjectConverter()

where the code tries to dereference the "contentBeansTool", which is not yet available.