code: public CrumbIssuer getCrumbIssuer() {
          Jenkins h = Jenkins.getInstanceOrNull();
          if(h==null)     return null;    // before Jenkins is initialized?
          return h.getCrumbIssuer();
      }
docs: Because servlet containers generally don't specify the ordering of the initialization
  (and different implementations indeed do this differently --- See HUDSON-3878),
  we cannot use Hudson to the CrumbIssuer into CrumbFilter eagerly.