Bug: droidbot gets unreachable UI element

[zhiyufan]

Hi, recently when I was using droidbot, I find that if I open an fragment within a screen, the get_possible_state is possible to get the UI element covered by the fragment. which makes the process slow.

However, when I use UIAutomator to dump the UI structure, the unreachable UI element would be marked by NAF tag.

If you need the test apk I used to reproduce the problem, I can send an email to you!