Summary: Previously, we would preallocate Node's with a child list of 4. We recently removed that logic (see diff below), and as a result, if you tried to access a Node's list of children before it had been allocated, you would crash. I added a simple check to protect from crashes, the operation of the check is O(1) so we shouldn't see a perf hit.
Reviewed By: emilsjolander
Differential Revision: D4104093
fbshipit-source-id: cd7b09818759aa76415b97e241f1a6746a2bc50c
