Intercept NPE in ActivityTransitionAnimator.

Note that the actual NPE happens inside GhostView extremely rarely
and I could never reproduce it. Regardless, it's better to avoid
crashing since there currently doesn't seem to be a path to prevent
it.

Fix: 402737819
Flag: EXEMPT crash prevention only
Test: manual
Change-Id: If7f97c9aeac2ceeac5218855ca7f6397efff9207