Fix two more issues related to reusing SingeltonContainer

1. If Paratext is opened early, then there will be no window,
so account for the active window to be null.

2. If the SingletonsContainer still has a Sparkle entry after restart
or on the start of a new application we will reuse it. We belive this
is safe since it isn't hooked into the application that was shutdown.

Change-Id: I79838d2f9f4c0a8c2fab1d3e278058b5b383730a