Fixes crash when getting panel before setting components

Update slippery in NavigationBarView may crash if panel view is null. At
view created the panel has not been set but does not crash on startup
because quickstep is not enabled then. However changing display size may
trigger it because quickstep is enabled. Therefore set the panel before
having any chances of updating slippery flag.

Change-Id: I129a181daa5f5088bef763327131f1a87a8a0cb5
Fixes: 115681967
Test: manual