Short Answer:
The life cycle methods have not run on app; so no createChildren() method was executed in order to create the child component of bt.
Longer Answer:
Things get slightly more complicated with the main application file, as it there is no higher level Flex component in the display hierarchy. I'm unclear on all specifics, but..
I think the Flex Compiler does some magic to set up this component--and the Flex Framework--that help makes the whole app work. You are, in essence, bypassing that work by creating your own instance of the component.