Viewstack and the other navigator containers use "deferred
instantiation" by default. This allows an application to load
faster at startup by only instantiating the visible elements. So
txtQuestion does not exist until a user navigates to that view.
The quick fix is to set creationPolicy="all" on the
ViewStack. This is not good practice, though. The correct solution
is to architect the app using binding or your own events to handle
the data availability.