Fix deletion bug in text-list-editor
Fix deletion bug in text-list-editor

Summary:
It looks like my backspace improvement broke this D:

Test Plan:
 * make a sorter
 * put three things in it
 * start backspacing at the end of the list
 * immediately after removing the last item, there should
   be only two items rendered (before this patch, there
   should be three items rendered)

Auditors: alex

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