add safe nav when setting `should_show_deeply_nested_alert`
add safe nav when setting `should_show_deeply_nested_alert`

fixes VICE-1972
flag=react_discussions_post

Test Plan:
- Hit the controller with no user and you should not get a 500

Change-Id: I546b4f24066eba1635870e54caa74715a0887098
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/272226
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
QA-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms