Fix WM shell crash when targets belong to a different display.

When showWhenLocked activity is launched on a second display, it's
possible that a window container for a wallpaper on the default display
and the showWhenLocked activity on the second display are both collected
for app transition.

This is a workaround to prevent NPE for such cases.

Bug: 260997053
Test: atest MultiDisplayLockedKeyguardTests
Change-Id: I9dbd0928aa785a6d547b8e19841748280ba982c9