Fix NPE going from PiP to Fullscreen.

Only happens when going to fullscreen when shell transitions are turned
on.

Bug: 271098548
Test: manual - make sure shell transitions are enabled, start a PiP, go
to the menu and select the fullscreen option --> app should go to
fullscreen and not just show a black screen.

Change-Id: I848b808182a0b0bae11401d74765cfd25e864d15