JS & page errors switching left nav items causes pages not to load (until refresh)

I went from testing NEXUS-7998 to NEXUS-7978 (Tasks to Users) via the left nav and I noticed two errors and the page doesn't load. Subsequent pages also did not load until UI refresh.
I was able to repro this after UI refresh but backwards from Users to Tasks and again from Users to Roles. Subsequent tests just showed one error however.
The JS console showed the below:

Uncaught TypeError: Cannot read property 'addCls' of null
:8081/static/rapture/baseapp-prod.js?_v=3.0.0-SNAPSHOT&_dc=1422566068855:1

Borderline major, I left this critical because it means refreshing the UI after every test and makes multi-page tests impossible.
I did not check NX2 at this time.

ADDENDUM:
Error slightly different in FF console.

TypeError: b is null
baseapp-prod.js:1