Traversal: fix bug when widgets are not present
Traversal: fix bug when widgets are not present

Summary:
This was presenting problems when adding a group widget, because
perseus one does an a11y traversal every time, and for whatever
reason the group widget doesn't have a `widgets` field defined
at that point (which we should probably fix, too, but this
code *ought* to be robust to people using renderes without
widgets, so this fix is still warranted).

Test Plan:
I can't repro this even in situations where i think i should
be able to repro this, so #fingerscrossed

Auditors: jordan

abe4mvp/perseus,ariabuckles/perseus,iamchenxin/perseus,ariabuckles/perseus,daukantas/perseus,learningequality/perseus,sachgits/perseus,alexristich/perseus,iamchenxin/perseus,chirilo/perseus,abe4mvp/perseus,ariabuckles/perseus,sachgits/perseus,alexristich/perseus,ariabuckles/perseus,chirilo/perseus,learningequality/perseus,daukantas/perseus