ietf-ipv6-router-advertisements submodule which augments ipv6 container from ietf-ip, causes build to fail with NullPointerException:
       Target type not yet generated: container ipv6

see attachment for stack-trace.

To reproduce this error use patch https://gerrit.fd.io/r/#/c/11658/.

The patch contains a lot of models, because it is hard to reproduce the bug, but it is definitely caused by submodule hc2vpp-ietf-ipv6-router-advertisements and its augmentation for ipv6.

If this module is changed to a regular module and used as import instead of include then everything works.

There are two cases, where the bug can be observed:

• first one shows the build failure (Steps 1 and 2) 
• second shows the same error but on run-time (steps 3 - 6),
  by deleting three unrelated models (dhcp@2018-01-03.yang, sfc-ioam-sb-pot@2017-01-12.yang, vpp-nsh@2017-03-15.yang) build succeeds but run-time fails.