[Crash] Fix case where node can be null and accessing last_message results in crash
[Crash] Fix case where node can be null and accessing last_message results in crash

artsy/eigen,artsy/emission,artsy/emission,artsy/emission,artsy/emission,artsy/emission,artsy/eigen,artsy/eigen,artsy/emission,artsy/eigen,artsy/emission,artsy/eigen,artsy/eigen,artsy/eigen