HTTP 500 when removing RIB config through RESTCONF

Tested: https://nexus.opendaylight.org/content/repositories/opendaylight.snapshot/org/opendaylight/integration/distribution-karaf/0.4.0-SNAPSHOT/distribution-karaf-0.4.0-20150805.055320-983.tar.gz

Steps to reproduce:
1. Configure RIB through RESTCONF as in: https://wiki.opendaylight.org/view/BGP_LS_PCEP:User_Guide#RIB_2
2. Issue DELETE on http://localhost:8181/restconf/config/network-topology:network-topology/topology/topology-netconf/node/controller-config/yang-ext:mount/config:modules/module/odl-bgp-rib-impl-cfg:rib-impl/example-bgp-rib

Hits HTTP 500 error. TRACE logs from bgpcep attached.