Fix crash when closing last app window

Fixing null pointer exception introduced by new a11y fix in ag/33576050
by adding better null checking, and putting behind original flag.

Bug: 422352975
Bug: 398732993
Test: DesktopModeWindowDecorViewModelTests
Flag: com.android.window.flags.enable_desktop_app_header_state_change_announcements
Change-Id: I625f5242823c83c2e5cd8a27003246ee66d35ee8