Fixes #25467 - The menu and top bar will disappear periodically

I haven't been able to consistently reproduce this, but myself and
other devs have seen the menu and top bar not reload on page refresh.
It seems to be correlated to an error found in the JS console:
Uncaught TypeError: Cannot read property 'firstname' of undefined

This commit adds a check that user.current_user.user is defined
in addition to the other checks.