CB-2458: gracefully exit with back button

If users do extra initialization, we can get NPEs when hitting the back
button before loadUrl() has been called.
-Null fenced code in startOfHistory() that gave us an NPE when hitting
Back button with useBrowserHistory=true
-Call finish() in Back button code when no history since with
useBrowserHistory=true it would just hang while the app inits
-Call loadUrlIntoView() first in handleDestory() since with
useBrowserHistory=false, the default behavior would try to use the baseUrl
which is null