Add a null check for getWindow()

Protect against crash surfaced by monkey, where it seems that the window
of the dialog is no longer present. If not present, don't need to post
dismiss runnable.

Bug: 124327419
Test: adb shell monkey -p com.google.android.GoogleCamera -s 999 -v 20000
Change-Id: Ic03b8501bb73725e04544fcb2397532be531988e