Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
if (!g_chainstate.RewindBlockIndex(params)) {
        return false;
    }
    if (chainActive.Tip() != nullptr) {
        
        
        
        CValidationState state;
        if (!FlushStateToDisk(params, state, FlushStateMode::ALWAYS)) {
            return false;
        }
    }
    return true;
}
void CChainState::UnloadBlockIndex() {