Add a null check in case device disappears.

This is a race condition possible when the caller get display IBinder
the display is still here, but when it tries to take screenshot with
that display, the display disappeared before it.

Bug: 75324434
Test: smoke tests on walleye. This is a race so hard to verify.
Change-Id: I5a107862e0b442840038b0956736e9661f556a23